By prime do you mean 168 RJ? Or do you just mean RJs one appearance at HW? Or some mythical version of Roy Jones where he kept his form from the lower weight classes and reigned as a HW? I think 1998 Foreman knocks him out, but it would be funny to watch him try to close the distant and eating shots along the way until he connected.. But once he did Roy Jones would collapse in the corner and fail to beat the count (might not even try to beat the count )
Good thing for Roy is he actually had the ability to fight a perfect fight. Really, really old George I'd give him a good chance. The George that lost to Evander or the one that beat up on Qawi would knock Roy senseless (even prime Roy)
I believe Jones can win a decision if he evades Foreman for the entirety of the fight and gets hit very few times. Jones chin will not stand up against Foreman's power. Even worse, Jones won't have the power to make Foreman respect Jones punch. Foreman was slow in his later career but I think this was strategy more than ability, sure he can't stalk Jones around the ring for the whole 12 rounds but Foreman was far, far, far more patient in the ring in 90's and knew how to reserve his energy. I see Foreman as a constant threat that I am unsure even a prime Roy Jones can successfully out point and evade for an entire fight, Jones will struggle the entire time to hurt him and Foreman will be patiently waiting with a counter or exploding on Jones when he gets too close I imagine.
If Roy Jones Jr. receives it, from a fight with John Ruiz that he entered with 193 pounds. And George Foreman from 1998 with 49 years, and when he had already finished his "second career". I predict the victory of Roy Jones Jr. by unanimous decision, because it is too fast. Something like a fight David Haye Vs Nikolai Valuev (without chicken dance).