I'm afraid I'm not aware of any Citizen specific repairers. I would think though that a good watch maker could work on the hand winders and automatics - the problem with vintage pieces though would be spare parts. They seem hard to come by. Someone willing and able to work on the vintage chronographs may be a different matter - I understand the design of these makes them more difficult to service / repair than Seiko chronos, plus the spares problem of course.
However, someone like Lew Brown is in a better position than me to give an informed view from a repairer's / restorer's point of view,