Sorry, I'm not much help when it comes to motors, but I'd CAREFULLY!!!! check the wiring and if that seems to be OK, I'd install a different motor. I think that's the easiest and cheapest way to fix this.....
Look for a 99 at yr Goodwill or online at
http://www.shopgoodwill.com/listings....asp?catID=415 in your location so that you don't have to pay shipping and then use one or the other as part machine....
All the vintage Singers that I have tried so far make a lovely stitch.