Arguably the 2 best Heavyweights H2H, before Joe Louis came into the division. Who takes this in a 15 rounder? Jack Johnson is the stronger man he has about 16-18lbs on him and he could smother and beat him him up in the clinches. At the same time I think Tunney is capable to avoid the clinching with his superior movement and footwork. Imo Tunney wins by a UD. He has the better technique, a great jab and very good timing. He was ahead of his time.
1910 or 1926 rules? Or some other period? Not that there's a huge gulf between 1910 and 1926, but the later you go, the more normal the conditions become.
It'd be hard for Tunney to beat Johnson without size advantage. Jack was very smart boxer himself and he possessed big strength advantage. I'd bet on Johnson by decision, though in a close and boring fight.
I'd lean toward Tunney just based on their literal performances in their respective eras. But that's the problem with such comparisons. One can say that Johnson never engaged anyone with Gene's mobility and hand speed and that can be true. So we only see/read Johnson doing what was necessary to prevail over the competition at that time. Johnson himself was fast and could move a bit on occasion when it suited him. Based on his natural attributes and relatively innovative techniques (which do arise in the face of the available competition, so the real skill is "adaption") , I could perhaps extrapolate Johnson into certain, more viable modifications (which I think he's more than capable of). I mean, he'd have to increase both his own mobility and output and be willing to put himself more at risk in order to get more punches home. He may not ever be disposed to same, though durable and rugged, Jack seriously didn't like getting hit, but it's not such a paradigm shift as to be impossible. But, exactly as he was, yeah, Gene takes the points but it's no runaway IMHO.
To what degree did Johnson show a stylistic evolution in the film we have of him (as boxing was evolving around him)? That might give you some of your answer. Although even then, it's famously hard to re-engineer a boxer once he gets fixed in his ways. Even if he understands intellectually what he needs to change, the muscle memory has been invested in a different system.