I give Marciano more of a chance against old George than young George, but I think Liston is the only sure thing in this scenario. Charles, Moore and Walcott would be slick enough to give old George some trouble and take him deep into a fight where anything could happen, but it might be difficult for them to completely avoid his power. Patterson is arguably my favorite champion ever but he has no business in the ring with young, old or middle-aged George.
Machen, Charles, Walcott, Moore and Liston. Folley has a good chance. Charles and Walcott were both past prime by then but Foreman wouldn't be too fast for them.
Hard to say as that version of Foreman never faced anyone as slick as Archie or Ezzard, no one with as much relentlessness and come from behind KO power as Marciano, no one Foreman fought tried to do much body work on him, they mostly tried to out box him due to the disparity in speed but often had moments of F-ing around and finding out because of Foreman's timing and because of his power it cost them Liston is the safest bet