He didn't lose his punch power nor his size advantage over Marciano and anyone Tyson had a size advantage over, normally never made it out of the first round. And Marciano is not making it out of the first round.
This would be Tyson's easiest fight. Marciano is nothing but a skinny, midget with sub-standard boxing skills who only beat midget boxers and old men.
Old Tyson was still a dangerous fighter. He never lost his power, but it was the sharpness and technique that Cus drilled into him that made him so dangerous. But that's what missing here. He's basically "just" a powerful brawler with a good chin and great power. I'd say that he could still beat Marciano on his power alone, bearing in mind that Rocky has to come into him. That said, I think Tyson would blow his wad early, and if he does, Rocky puts a fearful beating on him. I'd say 60/40 in Marciano's favour if we're talking about post prison Tyson. If Mike doesn't blow Marciano out early, I think Tyson was tough enough to see it to the end, and come out full of lumps for his troubles.
Old Tyson was the guy that was getting beaten up by Francois Botha. No disrespect to Botha, but I think Marciano is a cut above.