Scholars have absolutely discussed religious practice of slaves and ex-slaves in the New World, at great length in fact. You are right that it does not feature as much in popular discourse, though that is perhaps because discourse is very limited to issues reflecting modern concerns and I wager most African Americans are more concerned about economic inequality and racial prejudice than a distant ancestor's religion. There are, of course, groups who have been very concerned about cultural or religious change - the Nation of Islam being one of the more notable (and controversial) examples.

But forced conversion as you frame it was not a widespread characteristic or implicit goal of slavery was it practised in the United States or its predecessor colonies, from where the western English-speaking world draws its understanding of the system of New World slavery. At the very beginning of the slave trade, the distinction between Europeans was largely conceived as being cultural - racial hierarchy was in its infancy in the 16th century, though the roots of what would become racist thought were there. Slavery was initially justified and rationalised on the grounds that African culture was degraded and inferior to European culture and morality; religious difference, between the dignified grace of Christianity and what was perceived as the aggressive decadence of African heathenism, was part of that early understanding. In that context then, there was little incentive in colonial North America to convert slaves to Christianity: to do so undermines part of the justification for slavery in the very early colonial period. Indeed, some early slaves were able to find freedom and emancipation by converting to Christianity and demonstrating with satisfaction the sincerity of their new faith.

As we go through the 16th and 17th centuries though, and we see the beginning of a belief on the part of Europeans that Africans are inferior on racial grounds that are inherent and unchangeable, the importance of the religious distinction in the construction of slavery begins to change. From the 1660s onwards, laws begin to appear in throughout the English colonies explicitly stating that someone is a slave by virtue of their heritage and not their religion, and that conversion to Christianity is not grounds for emancipation. Virginia is one of the first colonies to pass such a law (and one of the crucibles in which early racial ideology was worked out) and its phrasing explicitly highlights that it is addressing ongoing confusion as to whether conversion to Christianity means someone is no longer a slave (and clarifying that no, that is not the case). Even then though, Christianisation does not become a uniform phenomenon in the United States; it begins in the late 1700s, but does not take off in earnest until the 1830s.

What really changes in the 19th Century is that slave owners in the United States are constructing an aggressively pro-slavery ideology in which they conceive themselves as the stern but benevolent parents at the head of a family that includes their (inherently degraded and vulnerable) slaves, and Christianisation becomes one of the ways in which slave owners can play up to that ideal. It occurs earlier than this in the British Caribbean, though largely without planter involvement in that case. In both cases though, this is largely the result of efforts by Christian missionaries and preachers rather than planters themselves, and in the United States they were not nearly numerous enough (or clever enough, to be blunt) to successfully facilitate mass conversion, rather than slave owners going out of their way to facilitate it.

It is also questionable just how Christian the faith embraced by some slaves was. Whilst certainly a significant minority were practising members of established churches, many more practiced their faith in private - either on or near plantations, usually in the form of secret gatherings for worship. These gatherings would often include practices and ideas with distinctly African origins, even if they were performed and expressed in a Christian context. This kind of practice was certainly not the kind of conversion the white planter class had hoped for, but it was certainly widespread. Many of the religious practices - both daily and secret - of slaves that have been taken as evidence for widespread, earnest Christian conversion betray clearly African roots upon closer inspection. Significantly, we see these practices even on plantations where owners enthusiastically embraced the cause of slave conversion - which would seem to imply some measure of overlap between those slave church goers and 'dissident practitioners' on plantations.

This is much, much more pronounced in the Caribbean than in the United States: in the Caribbean entirely distinct systems of religious practice - like Voudou in Haiti, Obeah in the West Indies, Santeria in the Spanish Caribbean and Myal (and off-shoot of Obeah) in Jamaica - emerge from the outset of slavery, some as quickly as the early 16th century, rooted heavily in traditional West African practices. As time goes on, some of these - or elements of these - faith practices become blurred and amalgmated into new forms of Christianity (in Jamaica for example, Baptist Christianity becomes very popular in slave communities, in part because it readily absorbs West African traditions into its practices). The United States' slaves struggle to maintain anything as sophisticated and coherent as that kind of identity but they do nonetheless still impart their own traditions and practices into Christianity. It has been suggested that the loss of those practices perhaps reflects the fact that blacks were a minority in American society post-emancipation, and thus the pressure to adapt to a more 'mainstream' Christianity (and thus receive some measure of cultural legitimacy) was greater than in the Caribbean - though in places like Jamaica and Barbados, many wealthier black or mixed race people would become Anglicans to try and fit in better with the white elite after emancipation.

I should that by 1850, every southern state had found cause to pass legislation restricting the religious practices of slaves, motivated by fear that rather than just being a means of social control or advancing southern white culture, religion could also be an inspiration for disobedience or revolt. Major religious slave revolts occurred in the United States, the Caribbean and Brazil in the 1830s had certainly shocked the white world; when the drive for Christianisation starts in the United States, many slave owners insist on a very limited biblical education that focuses only on commandments to obedience.

The story with regards to Islam is less clear and more complex. Only a minority of slaves taken from Africa, though a significant minority, were likely to have been Muslims - many more would have been exposed to Islamic cultural influences, mind. Obviously, the greatest obstacle to Islam's survival in slave communities is an inability to access the Qur'an; not only would the descendants of slaves from Africa been nearly universally illiterate in any language, but the Qur'an was not exactly an easily accessible text for anyone in the Americas - nevermind a slave - in this period. As a religion so strongly rooted in scripture and interpretative traditions of scriptural understanding, Islam had a very limited capacity to reproduce itself among the descendants of African slaves in a fashion that could still be authentically described as Muslim. More traditional West African religions, with greater emphasis on general spirituality, oral tradition and ritual practice, could much more easily translate across generational lines and appear much more frequently in the historical record.

What we do know is that some of those slaves who did come to the New World as Muslims continued what was clearly an attempt at Islamic practice. We have fleeting testimonies of their descendants discussing the way in which their parents or grandparents would pray and an attempt to reproduce some of the words they would say, which clearly indicate the continuation of Islamic practice. Likewise, there are fragmented writings by contemporaries that allude to Islamic religious practice, and perhaps the most famous assertion of Muslim identity in the New World was an 1835 slave revolt in Brazil, where some rebelling Muslim slaves reportedly wrote Qur'anic verses on traditional talismans. In general, though, evidence of continued Islamic practice comes to us only fleetingly through the historic record. We have no evidence of an unbroken tradition of Islamic practice surviving more than a generation or two after arrival in the New World, unlike West African traditions which, at least in the British Caribbean, we can still see legacies of today.

The contradictions between the realities of slavery and the philosophy of the Christian religion were not lost on those exposed to white religion, either, it should be noted. Much like southern planters had to construct a warped concept of freedom as their world came under increasing attack from abolitionism, so too did they have to construct a Christianity that not only sanctioned racial slavery, but the systems of repression and violence needed to maintain it. There is certainly evidence of slaves rejecting the faith outright, and of others expressing that they were essentially 'going through the motions' to please their owners, often because they recognised either the hypocrisy of the whites - preaching the love and forgiveness of God whilst keeping and treating slaves so brutally - or were frustrated at the lack of a place for their kind in white churches. Christianisation, where it occurred, was a power struggle in which slaves were active participants and agents of resistance; it was not a wholly one-way process of cultural destruction.