When most starters quit, they only need a clean-up of water or oil ingression and carbon dust deposits, their brushes and commutators are within specifications.
New starters cost hundreds $ and any parts could be hard to find. Servicing them with a bit of creativity could save you some money. Brushes could be somehow extended but the commutator segments have to be long enough. Brushes could be as short as 5.5mm not 55mm, my slip.
Do maintain electrical separation between the baseplate + housing + 2 bare wired brushes (ground side) and cable terminal + 2 insulated brushes + armature coil (battery positive terminal side).
Total service time on this starter: 1 hr