This area should be clean and dry. The brushes are compressed graphite, and will lubricate themselves and the commutator. If the brushes have gotten greasy somehow they are ruined, oil or grease turns the graphite dust into conductive tar that will basically short out adjacent commutator segments and reduce the starter's effectiveness.
You should clean out the gaps between the commutator copper segments with a stiff toothbrush and brake cleaner to get any carbon goo out. It it's really caked in then a metal tool can be used to scrape between segments. After that you will need to smooth off any ridges you created. This is simple, just cut a strip of 600 wet/dry and wrap it halfway around the commutator then turn the rotor to sand down the surface a bit while removing any high spots. Wash again with brake cleaner and dry, an air gun is good too to blast off any bits of sandpaper abrasive.
If you install new brushes it's a good idea to seat them before hard use. This involves running the motor for a half hour or so at reduced power (off the engine of course). A 2 or 5 amp battery charger should spin it OK. If the motor gets too hot to hold your hand on it stop and let it cool.
