Mike Tyson would have been able to defeat Muhammad Ali. These men have both won championships and are both very skilled in their leagues, but Tyson has used some punches and moves that Ali was never known to have used. Since boxing has become more advanced and some rules have changed, I think this would also improve Tyson's chances.
I believe Mike Tyson would have been able to defeat Muhammad Ali because he is the younger generation in the sport. When matched at their height of success, it may be less likely, but I would still put my money on Mike Tyson. Fighting was all Mike Tyson had and I believe he thoroughly understood how to win a match, probably better than any other.
Generally speaking, the more modern somebody is, the better they are than the past, often times simply because they studied the older group's techniques. Tyson came later and studied Ali among others, and both at their prime would have gone to Tyson, since he had far more knowledge and technique.
Mike Tyson was a great puncher and sometimes a pretty good boxer. But Mohammad Ali was both at all times for all but the last two years of his career. Ali was disciplined and hard working, while Tyson was lazy and easily distracted. Muhammad Ali beat some of the best ever and Mike Tyson had very little competition.
No, I do not thing that Mike Tyson would have been able to beat Ali, for a couple main reasons. The first is that Ali was taller, and would have a longer reach on Tyson. The second is that Ali would have been a lot faster, and that would have let him win.