if you were to be able to lower the motor maybe you could clear the chain stays and not need the tensioner on the motor side I use 10 speed tensioner on the crank side instead and while your friends house maybe you could heat and bend your cranks to clear your motor
I could lower the motor a little but not enough to run under the chainstay, and i quite like how it fills the space up. And i have a single speed flip flop hub so i dont have the 10 speed tensioner, but i could put a tensioner on the peddle side chainstay.
And im not heating BSA cranks..... NO WAY. It would probably damage the chrome finish a little. Longer bottom brackets are cheap enough