The easy answer here is that he would be badly outclassed, but I don't think this is necessarily true. The very motivated "Neon Leon" that beat Ali had very quick hands and feet, good upperbody movement, and great stamina and workrate. This was the kind of fighter that caused Ali the most problems in his career, not the big, strong punchers. What do you think, could he last the distance and nab a couple of rounds? Could he even make it close?
Ali would probably stop Spinks inside 8 rounds. His aggressive style and workrate would play into Ali's hands. I can see Ali opening up at mid-range with flurries. He'd move, create distance, and drop right hand leads on Spinks. A prime Ali is far too quick for him.
Although he had undeveloped potential, Leon was never ready for world class opposition, outside of his one performance against an aging,deconditioned and complacent Ali. Spinks was defeated by Ali at the near age of 37 in their rematch. In 1979, he was dusted in one round by Coetzee, and later destroyed within 3 by Holmes. Spinks just would not have the tools or talent to compete with a peak version of Muhammad. This would be a mismatch.
I'd reference the Cleveland Williams or Ernie Terrell matches. Ali in 66-68 was far superior to the Ali of 78. I think the outcome would depend on if Leon called him Clay or Ali. The former, a late, brutal humiliating stoppage. The latter, a merciful early stoppage.
It doesn't matter how motivated Leon is, he gets outclassed and punished badly before the fight ends inside of 8 at most. Prime Ali is simply on another planet than Spinks, there not even close.
I'm not going to say Leon would even look great, but the guy is underrated to some degree. When he was 5-0, he beat 27-0 Alfio Righetti, who wasn't a bad fighter in any way shape or form. Shortly after being psyched out by Coatzee, Spink's stops the equally if not more dangerous Bernardo Mercado. And shortly before that? Stops Evangelista, a decent B-level guy, who brought a younger Ali to a 15 round decision in a title fight. He wasn't THAT bad before drugs completely ruined him. Almost any fighter would slide as far as Spink's did if they followed his path.
Against a young Ali, Spinks wouldn't have had a chance. Ali would have done away with him in probably 9, 10 rounds. Spinks had the potential, but from what I understood, after he becam,e champ, he got caught up in the wrong crowd and squandered his winnings, his talents, etc, etc. His loss againt Coetzee was something not even I expected. Here's the fight: [yt]lmSv-CbrkVk[/yt]
He was diagnosed with the disease in 1984. IMO he may have had it as far back as the second Spinks fight. I watched an interview before they squared off for rematch and he looks drunk while talking. His eyes looked dreary and his speech wasn't what it once was either, although it wasn't particularly bad.
In Larry Holmes' book, Larry makes mention that his speech was slightly slurred and that before they fought, Holmes notices a SLIGHT tremble, but doesn't think anything of it. After their fight, the tremble becomes more noticable.