This may well incite derision 'cause the man doesn't get the respect he deserves on this site, partly because of his nationality, but Lennox Lewis would defeat a 70's Foreman.
Not only would he beat him, he'd knock him out before the fight went into the later rounds. A 90's version of Big George would fair better and could probably last the distance if the ref doesn't stop it late on.
In his 40's he wouldn't pose the same kind of threat so Lennox would be content to fight him at range, stay away from his right hand, pick him off with accurate punching and fight at a slow and steady pace.
In the 70's Foreman was a man too enamoured by his own power. Now while you could argue that he had good reason, it was at the detriment to his craft and conditioning. Physically and psychologically he'd be prepared to go five rounds. That wouldn't be enough to deal with a boxer of Lewis' calibre.
Lennox is bigger, has a longer reach, is naturally more athletic; had better movement; better hand speed; superior punch accuracy; strategically and tactically he was a better fighter than Foreman in the 70's; and he had a better grasp of the craft of boxing.
Also, Lennox would be feared half to death by the thought of getting caught by a man with George's size and power, and rightly so. When Lewis was afraid, he came out focused, sharp and willing to take his opponent out early.
I would never be so disrespectful as to suggest that Foreman would have had a puncher's chance, because he was a better boxer than that in the 70's so he would have brought more to the contest. He just wouldn't have brought enough.