I'd need to see the commutator in the housing to make a guess as to whether the brushes are too far worn to be worth trying to reuse, but otherwise you can lightly sand off any roughness on the brushes, and use a cotton swap with metal cleaner (brasso, etc) to clean out the brass holders. I'd also run the swab with metal cleaner, several times along the slot that the wire slides through, to not only clean but also polish it.
Remember that metal polish will continue to make black residue if you continue to polish brass, so don't skimp on doing a good job but you are not trying to keep cleaning till a swab with metal polish on it comes out clean because it never will.
After doing this, use a couple clean, dry swabs to get off remaining residue.
You might also stretch a "little" more tension into the springs in case they've been weakened by heat. If the motor bearings are sintered bushings, add light oil while it's open, to the *felt* reservoir cavities if it has them or to the shaft if not. Do not oil the brush holders.
