Nathan, I'm not saying an experienced technician couldn't remove the pinion mechanism without taking the whole starter motor off, but in this case you need to inspect the starter motor on the bench to see whether it has been trashed by running continuously.
Way back in my student days I recall a guy whose scooter's ignition switch (and everything else on the scooter) was worn out. He found himself 50 kilometres from home late one night and in a fix: the engine would only run while he held the ignition switch pushed slightly inward. (This is a very common problem when foolish people have had a heavy bunch of keys and dangly stuff hanging from their ignition switch for years). Unfortunately pushing the ignition switch inward activated the starter motor on that scooter. You guessed it: he rode it 50 kilometres while holding the ignition switch pushed in, so the starter motor ran continuously.
