A diamond tipped bit on a Team Cobra lathe would be worth every penny in your case. True the comm until it's a smooth consistent surface/finish on the face. Take an xacto blade and clean the slots while gently removing any burrs on the leading and trailing edge. Use a higher content silver brush with a serrated face. Give each edge of the brush a very, very light filing. Use a red spring on the positive side and green on the negative.
This will work very, very well for most any application. I have worked on many a brushed motor...more than I care to remember. The key was always starting with a very good base motor and then tuning it from there. Thanks to Team EAM (Eric Anderson), I always had great motors and brushes.