If you can conjure the two of them up and put them in the ring and Jimmy Young survives three rounds without getting knocked down multiple times and finally KTFO, I will eat my own excrement.
If Dempsey couldn't KO Tunney no reason to pick him to beat Young. Young if not better than Tunney is larger and probably considerably more durable.
Jimmy Young is not one of the top 50 heavyweights who ever lived, not even close. Tunney is easily top 20. Young gets props for "beating" Ali, which he didn't do. And for one lightning in a bottle night against Foreman. He is absolutely the most overrated heavyweight on this forum.
Prime Dempsey outworks prime Young for a decision win. Young fights to survive which wins him no fans among the judges.
One problem Jimmy Young will have is: When he ducks his head through the ropes outside the ring, Dempsey will reach over the top rope and punch him again anyway.
Dempsey by Decision, he comes out guns blazing and assuming he doesn't KO the extremely durable Young early, Dempsey's stamina allows him to continue at a high enough volume to off set some nice counters from Jimmy. It's a bad matchup in all honesty for Jimmy. This isn't Foreman with major stamina problems and punches you can see a mile away and this isn't Ali who didn't know how to lead and showed up in pretty poor shape himself for their bout. A lean 190lb and mean Dempsey works hard enough to win comfortable.