There are places that sell Bosch parts, and starter rebuilders who will
sell you a pair of brushes.
I've not had much trouble with anything but solenoids, which I have
repaired numerous times. The hard part is getting them apart. After that
it's not too bad. You have to unpot the screws on the end of the solenoid
and then unsolder the wires in the hollow brass rivets and you can
take the end cap off. On the 2 bolts that are contacts, the washers may
act like nuts so you have to grip them with something and unscrew them
to get the contacts out so you can smooth them up. After several iterations
you may want to put some silver solder (silver braze alloy) on them to restore
the original height.

I smoothed the commutator and made sure the brushes were long enough, or
replaced them. Never had a problem with the gear or the bearings so far,
but then it has only been about 30 years....

I have 3 starters that I swap back and forth, so I usually have a good one
to put in.
--
George Downs Bartlesville, Oklahoma