Re: Need help with some direction.
One thing you could do would be to squeeze the seat tube a bit to narrow it. That would allow the motor mount to fit. That won't compromise the tube's strength much is any at all since there are no side loads on it, it will actually be stronger fore and aft. You might have to get longer studs to thread into the motor. I found metric threaded studs at ACE hardware when I needed some to mount the motor to my mountain bike build. Just double nut the studs in the motor and unscrew them, putting in the longer ones the same way.
Goldwing 1500, Silverwing 600, Schwinn 49