Well, the answer is ...... yes.
Here is my 15-91 as I bought it Saturday before last:
Grungy, filthy, some rust, but complete and it turns fairly easily.
The wiring is toast. Most of the old latex insulation is missing from the motor and light wiring both outside and inside of the motor.
So since I don't have the proper wire or a grommet to do the rewire I decided to toss the motor parts into a tray and try an experiment:
I dug out my Edison an HA-1 / 15 clone and removed from it the hand wheel, stop motion nob, clutch washer, belt guard, motor, and light assembly.
Having already removed these parts from the 15-91 to see what I'd need to do to rewire it I decided to see if these parts would swap from the clone to the Singer and actually work.
They did.
Everything fit. Even the screws that hold the belt guard / bobbin winder to the machine's body threaded right in to the Singer.
The motor bracket screw did also.
I put a belt on it, plugged it in and tried it. It runs like a race horse.
So now I can answer the other members question from a day or so back, can you convert a 15-91 to a treadle or hand crank .... absolutely.
I've got to clean the machine up, it's still dirty, grungy and caked in ancient varnished oil, and of course I'll rewire the motor, but my curiosity is satisfied.
Joe