There are screws on the bottom of the machine you can back out to release the machine from the hinge pins. No need to switch hinges from case to cabinet or cabinet to cabinet. As far as I know Singer was pretty standardized in their hinge placement on their full size machines, except on the fiddle bases. If it's a Singer treadle and a Singer machine they should fit.
The bigger issue is the 15-91 has a built in (potted) motor. I'm not sure how that will affect trying to convert it to treadle. It might be easier to leave the machine electric and find a machine with an external motor to convert.
Rodney