Depending on how shop worn Foreman would have been by 1986, he either would have seen the writing on the wall with the rise of a young Mike Tyson and retired or he'd have stuck around and dueled Mike Tyson in a passing of the torch type of fight like say AJ vs Wlad
Spinks likely would have been stopped anyways, but if he had showed up in the same type of shape he was in for Cooney, he may have lasted a little longer. Spinks looked solid and like a heavyweight at 208 pounds.
And even then Tyson would be kayod in two rounds, max. Foreman had no fear of Mike, can't say the same vice versa.