Could you bend the rotor a little so that it takes a new path, just inside of the old one, where it doesn't hit any chipped area? Maybe put a little kink in the brass strip, or carefully surgically remove it, tin both cut parts, and overlap it back on, clamp with alligator clips, sweat solder it.
Another note: this method of volume control doesn't always work as nicely as the one used later in the audio section. Varying screen operation can sometimes get wonky and squawky. If you can get it to work so that it varies volume from zero to loud enough, great. Sometimes at the extreme it could go into oscillation and the instability may vary depending on frequency. Might not happen with your set and hope it doesn't.
Edit: Looking at your pictures again, it looks like the rotor tip doesn't ride on the carbon itself, but on a thin metal contact which gets pushed against the carbon? What I posted above might not work there. Hmm...
