Is this question still open?
Oh well, if it is, here goes.
The first thing I would want to know is: Is the (presumably title fight) 15 rounds, as in Ali's day, or 12 rounds, which is the norm today?
The second question to consider is: Which boxer is likely to hold up better over that distance? I am not a pugilist, but I imagine its different conditioning yourself and preparing yourself for 12 rounds and 15 rounds. I mean, you just can't plan to knock your man out---the knockouts don't always come against the higher caliber competition.
Third: We have something of a conceptual model, namely, Ali's three fights with Joe Frasier.
But then again, Mike Tyson in his prime, is a rather steep upgrade from Joe Frasier in his prime --- at least in my opinion.
Perhaps we should think of Mike Tyson, in his prime, as a shorter, more ripped, more disciplined, more technically sound, better defensively, and a sharper punching version of George Foreman.
Of course, all boxing fans know what happened with the "Rumble in the Jungle": a spectacular Ali knock out of Foreman in the eighth round ("rop-a-dope" strategy, and all that).
Would Mike Tyson, in his prime, trained by Gus D'Amato and Teddy Atlas, be likely to be tricked by such tactics? That is the question.
If Ali managed to stay on his feet, I would go with Tyson by 15-round decision.