Yesterday I removed the pair of motors and started work on designing the revised motor mount.
I'm waiting on a laser tachometer to arrive in the mail so I can measure the peak RPM of the motor at 70 volts. Then I can determine what reductions are needed.
The drive shaft of the motor unfortunately has a mix of imperial and metric dimensions. The tip is left-hand threaded 1/2-13, but the rest of it is 17mm.
I'm probably going to end up making a V-Belt pulley for it from scratch.
Edit: And now I have the motor mount finished.