I decided to take the whole thing apart, since it was full of grime and carbon.
After a decent clean, I lubed the drive, and put it back together.
Sourcing new brushes was difficult, so I raided the local wrecker. Bosch made starters for many different vehicles of that era, so eventually after dismantling a few similar shaped ones, I found the correct brushes. They have a bit of life left in them, but that's fine since this is only a temporary fix.
After final reassembly, I connected it to a weak battery I have laying around, and tried it out.Testing the starter.