I would say the guts of the gear drive are the problem. The gear drive housings can be disassembled and I doubt there's much inside of them, but I can almost guarantee the grease is as hard as a rock. A big clean out, some low melt grease and possibly a look at the motor to check the condition of the brushes/armature etc. This should be easier than stripping a starter motor down

*Graphite wiper blades are brilliant