That's going to depend a bit on the nature of the part and the desirability of the camera,
@Dralowid.
Making parts for a Leica III - well documented, relatively simple construction, fairly desirable camera? Easy with the right tools and know-how, makes sense economically.
Making parts for a Leica M3 - well documented, more complex construction, very desirable camera? Not too much harder or more unlikely if it comes down to it.
Making parts for an Anscoflex? Yeah, not happening. Not desirable enough, not documented enough.
Making parts for a Contax G1? Also not happening. Desirable enough, but too complicated and too many microelectronics.
I was watching a fantastic video about a guy rebuilding his dad's old motorcycle completely from scratch last night. In it he said the biggest thing he learned from his dad was that if someone built it, he should be able to take it apart, fix it, and put it back together. Same thing applies here: as long as the original parts were made by a machinist and not a robot, it wouldn't be too hard to recreate them if you have the tools, the drawings (or reverse-engineered ones), and the know-how. Once you get into the era CAD/CAM and automated production and assembly, you're pretty screwed.