According to Festus, he writes "Niger lapis in Comitio locum funestum significat, ut ali, Romuli morti destinatum...".
Or: “Niger, a place for an intolerable injury to a stone in the Comitium, signifies, that they may be fed, that marked out to death of Romulus”
So he’s buried under the stone and if we dig there that’s where we could find his tomb, am I missing something?
Wikipedia says “It is there too that a part of the tradition locates the place of his death”
https://en.m.wikipedia.org/wiki/Vulcan_(mythology)
Look under Vulcan and the foundation of Rome.
That translation is ... a little iffy to say the least. What it roughly says is "a/the black stone in the Comitium points to the sinister place destined for the death of Romulus". Or something more or less in that direction.
I'm sorry, maybe I do not understand your question correctly. Yet, Romulus himself is most possibly a myth.