I don't own any PEGs so I can't give you a ready answer, but if you had the gearbox out you could take measurements of the existing motor and that would at least allow you to determine the case size currently in use. And if you had it apart enough, you can count teeth on all of the gears and determine the gearing ratio and. That would allow you to model the speed and torque you'd get at the wheels for any given motor replacement. Then if you had a target speed you wanted it would be possible to at least evaluate motors against that criteria.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors