It would have been awesome. His size would have accomodated his MMA style too. Usually smaller guys have trouble with bigger guys on the striking but have a much easier wrestling base. Tyson would have been able to learn his wrestling base, and demolished everyone standing.
That being said, I do agree that Ali would have beat him, even though Ali is the most overrated of all time. Tyson doesnt try to cut of the ring, he charges. Joe Frazier could put Ali on the ropes and slow the rope a dope down, while Tyson would have been walking straight into jab/straight combinations as Ali danced around the ring. Also Ali took shots from Frazier and Forman, who are both disputabley bigger punchers than Tyson. Lastly, Tyson choked in everyone of his biggest fights. He made the majority of his legacy demolishing nobody fighters.
Anyways, yeah, Tyson in MMA = Win