Well the question asks until Ali won title from Liston, perhaps we ought to remember that in the fight prior to the Liston bout Clay as he was then known was put to the canvas by a 6'1 185 pound Henry Cooper, i guess who do we see as the tougher harder hitting opponent Cooper or the 6'6 240+ pound Willard. I of course think Ali beats Willard, but Ali was no Dempsey regarding ferocity of attack and Willard had good gas tank so distinct possibility he sees the final bell.
Ah. So FINALLY we get Concussion Proof WONDER WILLARD, with 50 more years of experience! A lot depends on if we can keep him motivated enough to keep him fighting the best fighters throughout those 50 years, win, lose, or draw. If we can keep him going, he would probably attract some pretty good trainers and promoters along the way. This entire scenario has money written all over it. If we could keep Jess motivated and learning along the way, he could probably have a few minor moments against Muhammad Ali and give at least a decent account of himself. I do think that Prime Ali's abilities would be a bit too much for Jess. Even Wonder Willard would have had his limitations. I'm going with Muhammad Ali by unanimous 15 round decision.
This man is no ones fool, Einstein would have struggled to conceive this prognosis, go to the head of the class buddy.
As an aside, I envision Wonder Willard, making his entrance down the aisle to the ring, riding on a HUGE animal.
Wouldn't a better question, be which is the youngest version of Ali, that this version of Willard would beat?