The rules would not matter. As soon as Fitz lands one of his best, it would be the beginning of the end for Roy. Roy Jones could not take Fitzsimmons shots to the head or body.
Middleweight Fitzsimmons needed 13 rounds of severe beating to stop natural welterweight shot Nonpareil. Big puncher, indeed.
Well, Fitz career was just getting off the ground. He was only 150.5 pounds for that fight. Nonpareil was a true pound for pound type of fighter. Fitz floored the game Nonpareil many times. As for Fitz power, he iced Maher, and Shareky early. He Ko'd many top contenders, and killed three men in the ring. From 1890 Rollins to 1903 Gardiner ( Fitz's best years ), Fitz had 40 fights. Let’s look at the numbers. Fitz knocked out 33 of 40 fighters, or 82.5%. While this number is impressive, it is misleading. Fitz really Ko'd Sharkey but lost via DQ ( 34 of 40 or 85 % ). Martin, Choysnki, and Grimm were 6 rounds or less matches. Fitz did not have enough time to knock them out. In the 40 fights, Fitz only two real losses in this 40 fight stretch were to Jeffries. The only man to go the distance with Fitz from 1890-1903 in a full fight was Gardiner who was floored 4 times. By 1903, Fitz was 40 years old, yet he beat up a very good fighter ( Gardiner was 39-4-2, only 1 TKO loss at the time of the match ) and won the light heavyweight crown. The all time leading KO king is Archie Moore with 131 documented knockouts. Much of Fitz career is undocumented. Fitz claimed to have over 300 fights. This is possible, and if true, I’d betcha he had at least 150 knockouts! I'd say Fitzsimmons could punch.
Roy Jones Jr with his speedy circling and leaping ability would give Fitzsimmons all kinds of stylistic problems, but he would not prove an unhittable enigma. James J Corbett incorporated a lot of lateral movement that modern fighters today would find very hard to emulate; he could really move, and while Jones maybe that little bit quicker, Corbett was a lil' bit stronger and able to deal with Fitzsimmons mauling tactics a step better. Jones won the majority of his fights through taming his foe, but Fitzsimmons has the ability to ask hard questions all night long. Things Bob has going for him is mental strength, persistence and a smart head. He had super technique and a great sense of timing. Against Corbett he showed how to lay in wait and then make it count. It's either a points win for Roy or a knockout win for Bob - both outcomes Ted Spoon views as even.