Let me see,,Parker was ko'ed by one of the slowest heavyweights of recent years in Joyce and he's somehow going to avoid the quicker explosive attacks from the Manassa Mauler.,,, Ref rescues Parker , Jack by TKO
Jack Dempsey at 187lbs against the much bigger Parker who can box and is 260lbs, Parker crushes him early...
Parker can actually box and is (was) an actual full time boxer. The only rodeos that distract him are Bolivian.
Willard didn't start until he was 27, and he did go 26 rounds with Jack Johnson albeit past prime but much more than anything we can say about Parker.
I'd pick more than half a dozen Parker victims to beat the version of Johnson seen in 1915, and to do it much more quickly than Willard.