Willard's size, strength, ability to tie men up on the inside, and incredible stamina would keep him in this fight and make winning a tough test for Baer. In the end however, Baer's relentless attacks and punching power would probably end in a KO win sometime late. I wouldn't expect this to be a chess match as neither of these men were terribly beautiful in the skills department. A very crude fight with a lot of wild swinging at times, and a fair amount of wrestling at others. probably a crowd pleasing match though. On a sidenote, I think Primo Carnera vs Jess Willard might be an interesting one.
Baer is too ferocious. Willard is very average, maybe even worse than average in regular 12, 15 round fights. In fights to the finish though, which the Johnson one was one essentially, he was very good obviously due to his size and durability.
Baer by a 12 or 13 round tko, with 2 knockdowns sending Jess crashing, causing the ref to intervene. Don't underestimate Max and that right hand against a giant.
Willard was durable and could possibly outlast Baer,but Max went 20rds to win in some fights,and Willard was not hard to tag for someone with a good reach like the long punching Baer.Max by late stoppage.