You might want to give Goopy Performance a call. They have a rotor housing refinish/refurbishing process that is described in this thread:
http://www.rotarycarclub.com/rotary_...ad.php?t=13408
From what I read, the process appears very promising for 13Bs. I have no idea if Goopy can set up his tooling to do anything with your NSU housings, but it wouldn't hurt to ask.