I'd say Ali would win.
Foreman had a punch which was as powerful (if not more so) than Tysons, Ali took him on and went round after round against him. He used his movement and smarts to simply outwit and out play Foreman. Like some of the great boxers of today like the mayweathers and the pacmans, and before them jones jr, they excelled at not being beaten by following the mantra that you cant beat what you cant hit. Another thing to remember is that Ali could take a punch, something which I think many people forget.
I'd say that Ali would out smart him and wind Tyson out, its a classic style vs power fight and as we've seen many times in the past quite often the power player in those matches ends up losing to the smarter style boxer.
(I also actually think it would be a pretty awful and dull match to watch)