Hard to say, the obvious candidates would be Moore, LaStarza, Layne, Harold Johnson, Valdes, Baker. I'm sure he beats some of them, I think he beats the big slow guys, but does he beat them all? Not so sure
Layne seemed to have a lot of success against an admittedly out of shape Louis in a boxing exhibition.
I would favor Louis over all of them, except perhaps Moore. I would even favor Louis slightly over Johnson.
In 1951 Joe Louis was fighting regularly and receiving favorable writeups from sportswriters. He had been aiming for a September 1951 rematch with Ezzard Charles when Jersey Joe pulled an upset with his KO of Charles. Since Walcott and Charles would rematch, Louis signed to fight Marciano. He was probably in the best shape of his comeback when he fought Marciano and was in fact the betting favorite going into that fight. Therefore, I would pick him over all other contenders at this time, although LaStarza and Johnson were gaining on him fast. A match with Moore at this time would have been very interesting.
I dont see any one except the three already mentioned beating Louis...Moore has a chance...so has la Straza...though I think both gets beaten probably.