The Bridgeport motors are proprietary C-face motors. Any other motor single or three phase would have to have a mounting plate to fit the top housing.
I don't know if you could do a retro fit with just an adapter plate.
Being, as you stated it's a proprietary motor the shaft is going to be quite unique not just the mounting of a different motor.
But we are talking about machinist here and they can work wonders and sh it cucumbers when it comes to creating custom parts...but he's got to get the mill running first...
I replaced a motor on a mill like that once probably ten years ago, it was a direct change out of a smoked motor.
If I recall correctly the motor cost was over 3 grand.
