I hate to see motors end up in an early grave, so I gotta ask: with the belt removed, does the machine spin freely by hand? And by freely I mean when you give the hand wheel a spin it should go for multiple revolutions before it slows down and stops. When you spin it by hand, is it stopping at the same point in the machine cycle each time? I just finished a 66 and had to adjust the feed dogs ever so slightly as there was some binding and the machine stopped in the same spot every time I spun it up. Does the motor spin freely detached from the machine when you power it up? Don't step on the gas and stay there but see if it spins up by itself.
If the machine spins freely and the motor spins freely, you may have a belt problem. You should have a proper v-belt to test it. I posted about the World's Best Sewing Machine, which I happen to own, and the motor ran well, the machine was smooth, but when I hooked it up with the cogged belt that came with it I couldn't get any RPMs, but with a new v-belt and the tension just so, it took off the way it was supposed to.
And if you end up ditching the motor feel free to send it to me!!