I bought a 1952 Ford 8n tractor that supposedly had a locked up motor, the first thing I did was to remove the starter. When I had it loose, I heard a pop and the motor was free. The po tried to pull start it, and the wheels would lock up when he released the clutch.

I know the starter is a pain on a R107, but I would remove it to make sure the bendix isn't bound up on the flywheel.