There are two groups you might join to find resources for parts: needlebar.org and the Yahoo! group Vintage Singers. Sometimes you can find parts on e-bay.
I was in the market for a 15-91 a couple of years ago and ended up buying an already restored machine from a poster that I recognized. There are folks who are interested in restoring machines themselves as a hobby, so you could find a motor from a collector who bought a machine with for parts.
Good luck!