I ran into a guy who owned one of the service companies that used to supply rebuilt CRTs for televisions... it was a good business for about 15 years. Basically, he would rebuild (install a new gun assembly) onto a unit with a failed one (either filament damaged, or emitter components damaged)… he did not do any re-phosphor. He could charge T.V. repair shops about 20% less than the manufacturer supplied CRT.
My guess is... if the guns for Tek CRTs are not available (and where would they be?)... or unless possibly one wanted burn-in repair on the screen (so re-depositing the phosphor) on an otherwise strong CRT... then rebuilding is not going to happen. Guns for popular television tubes where manufactured by the 100's of millions... and maybe still are available in China, of the former C.C.C.P. countries. Tek guns... not so much.
Best cheers and wishes.