If you are based in the UK, I can highly recommend Dave Fisher at Moathouse Magnetos in Dorset. He produces good thorough rebuilds (not just a new lick of paint like some) at reasonable cost. He also has replacement armatures etc and can source complete mags for you. His waiting list is about 6 weeks for a rebuild though due to the demand for his services. Hs address and details are in the usual mags OBM, Classic Motorcycle, VMCC Journal etc.
Regards,
Martin