Howdy
Really depends on how much "work" has been done to the gun. Sometimes they have been "modified" beyond repair. If that is not the case then I guess Robbie Barkman is as good a place as any. In the past he has been reluctant to talk about or supply info on his involvement with Detonics, but you never know. He is a good smith and nice guy to talk to. Also you might want to try TJ's custom guns as I think he has some experience in the "New Detonics" area. I don't think the Gunsmith's at Detonics USA will want to work on it but you can try. I have worked on many Combatmasters but I don't do mail-order work.
Hope this helps.....
L8r
Conley Janssen
USMC87-93