French bike parts actually are not that difficult to find. If you want to upgrade the cranks to cotterless, you can buy new French cups and bearings from Action for under $15. You would need the spindle and cranks to complete it. The headset could be addressed with parts from a standard threaded headset. The cups (bottom only if possible), bearings, and crown race (if 27.0mm) from the new headset are used with the top race, spacer, top cap of the old headset. You can also buy a new headset from Velo orange if your crown race is 25.4mm. Definitely reuse the stem, bars, seatpost as these are going to be sized differently than modern parts. The 126mm spaced rims should squeeze into the dropouts without too much effort. Any other "problems" can be addressed without too much cost.

The project is definitely doable with lots of research and careful measuring of the parts you have. Sheldon Brown wrote about French bikes, as did BF member Randyjawa on his site Mytenspeeds. I would strongly suggest you read up before you jump in.

While not the most economical way to get a bike, working on French bikes has other rewards. The challenge of learning new things, the pride of saving a bike that will never be made again, and having something different than the average guy can make it worth it. In addition, lots of people love the ride of French bikes, even the lower end ones have decent riding characteristics that other lower bikes don't.