It's a pretty simple job. Usually new brushes, cleaning the old brush dust and cleaning up the commutator will give new life to a starter. Cleaning and relubing the planetary gears on the business end will help too. Be sure you keep track of the number and type of washers and wave washers on each end of the armature shaft. If you don't want to do the job yourself most automotive electrical shops will rebuild the starter for you. If you do a search on the forum there's quite a bit of information on rebuilding these things.