More than likely the belt and motor has nothing to do with it. As ArchaicArcane said, remove the belt and try it then.
You might not be able to see it, but there still could be a piece of thread behind the bobbin assembly. That was the case with our FW when we got it. Locked up tight. I had to take the bobbin assembly apart to clear it. Made all the difference in the world.
Then oil it. If it's stiff and rough it could just need oil. And grease.
Joe