Jai Opetaia and Briedis, but those guys are over 210 pounds on fight night so they're just smaller heavyweights in disguise The best version of Bivol, Michael Spinks, cruiserweight Holyfield and heavyweight Roy Jones could hypothetically outbox Marciano.
I would say a Cruiserweight Evander Holyfield has the best chance of beating Rocky Marciano, assuming the multifarious Evander shows up with the right style and sticks with it for the entire fight. He did not always do those things.
Those cruisers would have been heavyweights back in rocky's time. I feel like picking a cruiserweight is skirting the limits of this hypothetical scenario. I suggest Qawi. If nothing else i'd be funny watching Marciano fighting someone shorter than him.
Lloyd Marshall beats him. He beat Sheppard. Ketchel beats him. If we can include HW combo fighters this can be a long list.
Ketchel died for your sins. And yeah early film gets degraded over time. You can't expect it to look pretty.
Holyfield M Spinks would tear up Rocky and probably Qawi would be a clear beating as well. Many of the great light heavy’s of the eighties are possibilities.