If the metal plate still has any original paint on it you can tell if it is 6 or 12 volts.
6 volt has a black background.
12 volt has a red background.
6 and 12 volt starters can not be mixed on flywheels.
You may have a bad Bendix but if that is not it than it could be the flywheel teeth.
It is common for the leading point of the flywheel teeth to become ground flat so the Bendix teeth spin on the end of the flywheel teeth without engaging.
This is especially true on a 12 volt conversion where the 6 volt starter was used.
It helps to sharpen the teeth on the front/engaging end of the flywheel so the Bendix teeth engage easier.
