Does the vehicle still have a 6 volt electrical system? If so there's a definite possibility you have the original (6V) starter trying to engage a 12V flywheel- - - -No Bueno!
If you have access to the bottom of the bellhousing, remove the sheet metal cover, mark one of the flywheel teeth, and turn the engine slowly while counting the ring gear teeth. A 6V ring gear has 139 teeth- - - -a 12V has 168. No, the ring gears are not interchangeable, but a 6V flywheel will bolt onto an engine that originally came with a 12V system.
Jerry
