36 yr old versions of Joe Louis and Muhammad Ali fight each others opponents. (For Leon Spinks, well say he's Spinks from Ali-Spinks I; and ofcourse this is the Charles who defended against Louis; pick which ever Ali (f/ Spinks I or Spinks II) you think has the better chance).
The ageing Muhammad of both Spinks fights drops a clear decision to a prime Charles. The ageing Louis still has the punch to halt Leon Spinks.