I think the common answer is Joe Louis, so i was wondering if there are more who would've been capable of doing incredible things too regardless of their size My nomination is Usyk.
Most of the greats then and I say this even though I am sometimes viewed as a heavyweight hater. They already have smaller fighters you could compare them to anyway. Marciano to Moon, Ali to Legra, Louis to Napoles, etc. Obviously those aren't 1 to 1 in terms of greatness but if Legra can have success I see no reason why Ali couldn't have success if shrunk down to 126. This is a made up number I'm making up but I imagine if you're around top 20 all time in a classic division you could have success in any weight class in any era, it might not be the amount of success they had in their own era but they wouldn't be a bum or something.
James Toney and Roy Jones. I have a feeling Michael Spinks style would work pretty well but just a feeling.
Holyfield, Holmes, Usyk, Tim Witherspoon A controversial pick would be Lewis. If Lewis still had a height and reach advantage with KO power over most of his opponents like Hearns his style can translate to lower weights, Lewis could fight at all ranges and his inside fighting skills get underrated