I suppose it depends on what all is wrong with your current one. I've looked over the shoulders of guys rebuilding them in the local shop here before it closed, and for the most part it didn't seem that difficult, but you may need some tools like a lathe that aren't in everyone's garage. There is also some electrical test equipment that makes figuring out if the windings are good easy. If your windings are good and all you need is a set of brushes and maybe some bushings it would be a fairly simple job. The lathe comes in if the contact area for the brushes is damaged and needs to be refaced.
I'm not sure how the economics works out if you need more than just the brushes and bushings, but I had mine rebuilt when it died a few years ago, and it needed pretty much everything internal replaced. (it had been rebuilt before, probably more than once and failed the electrical tests for all windings) If you can get the parts, putting them in wouldn't be that bad either, but I had mine done at the shop.
