Like the 401 series, the motor is a "Sealed bearing", no grease pots on this one.
That said, there are brushes, so those are worth looking at. I've had motors with the brushes worn down to nubs, and they stop working completely. The brushes should be 1/4" or longer. Brushes can be checked by removing the bottom panel of the machine and looking for the "springs" in the motor. The grey part to the inside of the springs are the brushes.
The other thing worth trying - Assuming the motor just growls and doesn't move when you press the pedal:
there's a bearing on the top of the motor. If you remove the top of the machine (as though greasing the upper part of the machine), you will see a fiber gear attached to the handwheel, and a "worm gear" that it rides against. Follow the worm gear to where it disappears into the bowels of the machine... this is where the bearing is. It gets gummed up with years of that grease seeping down there, and seizes solid.
I find usually about 5 drops of tri-flow oil for a seized motor bearing (about 2 for a regular maintenance thing), let it sit a few minutes, then gently see if the handwheel is willing to rock back and forth a little. It will loosen up pretty quickly. Once its free, you should run the oil into the bearing by running it for a few minutes. Some people say full out, I usually just run it.
If it's really kaput... the motors from 401s, 403s, 500, 503s and I think some of the 600 series machines will work if you can find them.