It's a piece of cake to make a treadle or a hang on motor machine out of a Singer 15-91.
I've done it as an experiment.
First, disconnect all the wiring.
Second, remove the hand wheel.
Third, locate and remove the two screws holding the potted motor assembly on.
Fourth, clean the front of the machine.
Fifth, acquire a belt guard/bobbin winder, hand wheel, stop motion knob and washer, and motor with mount and light if you want, from a clone. The clone's screws that hold the bobbin winder/belt guard on will fit the Singer.
Sixth assemble in reverse order, plug in and sew up a storm.
If you go to Sew-Classic she has the belt guard and hand wheel for the conversion to hand cranks. She may also have the screws. That's really all you need to convert the Singer for treadle use.
Here's my 15-91 converted to be a Frankenstein machine:
The loaner machine is in the rear.
The two screws needed to mount the belt guard/winder should be available from most Singer repair
shops.
Joe