Don't undercut your starter commutator, for the reasons Jerry said. Generator commutators with carbon brushes routinely are undercut.
As for the bushings, you should be able to drive out the drive-end bushing with a punch. There are bushing-driver punches in various sizes to fit different bushings, but you probably have to buy a set. You may be able to remove the commutator-end bushing by threading a tap, about a half-inch, I believe, into the bushing, clamping the drive-end of the tap in a vise then tapping the end frame lightly to separate it from the bushing. Then use a punch to install the new bushing into the end frame. Tap lightly and evenly around the circumference of the bushing to seat it in the end frame so as not to cock it in the bore, or break the bushing.
Solvent won't hurt anything in the starter, except the over-running clutch part of the drive. Washing the drive will wash out whatever lubricant remains inside the clutch and cause the ramps and rollers inside to wear prematurely. You probably should replace the drive- that's normal practice when repairing a starter.
Check the shifter fork for wear where the bearing surfaces run in the drive channel/sheave. If the bearing surfaces are worn, you should replace the fork, but in some starters, the fork can be reversed so the unworn part rides in the drive. I don't remember if that will work with your starter. When you assemble the starter, put a little grease on the bearing surfaces for the shifter and on the armature shaft where the drive rides.
Harvester
