I have been struggling how to respond. I am going to focus on the daily life of an observant Catholic who follows the teachings and rules of the Church. If you want me to answer in another way, let me know.

St. Augustine would be completely shocked at how little the Church fasts nowadays. The ancient Church generally had three major fasts: Great Lent (today's Lent), the Apostles' Fast (roughly 40 days in the middle of summer leading up to the Feast of Sts. Peter and Paul), and St. Philip's Fast (40 days before Christmas). (There was also a fast for 15 days before the Dormition of the Blessed Mother on August 15). This is also in addition to fasting on every Wednesday and Friday of the year.

Additionally, the ancient fast was much more severe. A typical fast in Lent in the ancient Church would be no meat for 40 days (except fish). There would also be days of vegan fasting, like on Good Friday.

However, with the help of the translator, St. Augustine would (hopefully) not reprimand an observant Catholic for not fasting as much as an ancient Catholic. The reason for this can be found in the common phrase "When in Rome, do as the Romans do." This phrase actually comes from a conversation between Sts. Augustine and Ambrose regarding how to fast when visiting Rome. Local churches in the ancient Church had much more variety in fasting discipline, especially with regards to the fast before Communion. St. Augustine asked St. Ambrose (who had helped bring Augustine into the Church) whether he should follow the fast according to the custom in Hippo when traveling, or the fast according to the custom in Rome. St. Ambrose's suggestion is to follow the custom wherever one find's oneself.

As for theology, this is a more tricky question. Catholics, Orthodox, Protestants, Evangelicals, Non-Denom, and any other stripe of Christian you can think of tend to claim St. Augustine for their own. I will note, however, that St. Aquinas (arguably the most influential theologian in the history of the Church, and certainly the most influential after his lifetime) relied heavily on Augustine. To the extent a Catholic follows the teaching of the church, they are largely Thomistic in their theology and by extension share much with Augustine. I would argue that a Catholic who believes in the major dogmas of the Church would find fundamental agreement with Augustine.

The major differences theologically between a contemporary Catholic and Augustine, IMO, would be emphasis. The theology of the early Church and the attendant controversies focused much more on the nature of Christ, the nature of grace, and salvation generally. Catholic apologetics today focuses much more on either Protestant and/or Orthodox critiques of Catholicism: e.g. birth control, female clergy, sola scriptura, nature of the Eucharist, devotion to the Saints/Mary (I will note one of the Ecumenical Councils of the early Church [Ephesus] focused heavily on if it was proper to call Mary the Mother of God), and papal authority.

In the early Church issues like sola scriptura simply did not exist. Other issues, like female clergy, the Eucharist, Saints, were actually areas of pretty general agreement among all Christians. The nature of the papacy stands out as the only issue whose theological history does not really erupt until 1054 A.D. (with some apologists pointing to the Accian Schism of the seventh century as perhaps the earliest papal controversy/clear teaching), and off the top of my head I can't think of anything St. Augustine says particularly on this issue.

So there might be some theological language issues between St. Augustine and a modern Catholic, e.g. transubstantiation when talking about the Eucharist, but I think on the whole he would find a contemporary, believing Catholic to be a recognizable Christian.