Wooden racquet and no-jump serve (i.e. one foot always touching the ground during the serve).
Also, it probably wouldn't be easy for 6'10'' Towers to play as much (grass) matches as shorter 5'8'' guys would (without serious or niggling injuries), and they might have problems with a recuperation between the matches and between the very games during the set (no chairs during changeovers).
They didn't play tie breaks back then, something Isner is a specialist at.
Having to win every set by advantage may be harder for Big John.
But playing comparatively short players like Rocket and Muscles, I think he'd cause them a lot of trouble.
Interesting discussion, but very hypothetical.