How fixed? For serious internal work, I would try either Colt or Cylinder and Slide (
www.cylinder-slide.com). There are very few gunsmiths today who understand the old DA Colt actions, and I would hate to take someone's word on his capability without some backup in the way of testimonials from past customers. Some people say even Colt has forgotten how to work on those, but a phone call won't cost a lot to talk to them.
Jim