re replacement bits. If you feel like swopping bits around you may be able to find all the unthreaded components from an english headset will fit together leaving just your upper cup and locknut. If the upper cup is not too bad this might be one way round unavailable french headsets. Be careful if you go into the NOS or used market, there are a number of headsets kicking around for french bicyclettes from the late 50's and early 60's which bear no ressemblance to the ones fitted to quality machines. If it has a knurled locknut beware!!
Rhil Wood used to do a bb that had a number of modulable cups for the different thread standards. They were on Harris Cyclery (Sheldon Brown's commercial outlet). Otherwise there are also bbs that are designed for knackered threads and tighten on themselves. Stronglight used to do one that required the entry to the threads chamfered at 45° so that it lined up when tightened on itself (from memory that is what it did). Otherwise rethreading to italian is the easy option to getting a choice of modern bbs (same thread pitch, same width shell just 36mm in place of 35mm diameter cups). In my experience it is the axles that pit before the cups so find a replacement axle and replace bearings will recover things. Note though that french standard axles are not the same as english ones. I cannot answer for italian ones in this respect, they might be and might be a lot easier to find. French standard Campag cup and cone bbs seem to be virtually indestructible (like their pedals also).
Pedal threads can be retapped very easily to english but given that french thread pedals may be among the easier items to find it seems a pity to do this unless you want to use non-original pedals (automatic for example).
Afterthought I have an italian bb axle somewhere that I could measure and compare with a french one. Won't take me more than a couple of months to find it in all the junk
Afterthought 2. Some french LBS that ought to know better replace french headsets with english ones with aluminium cups. The ally recuts its own thread and gets the bike out of the shop. Doing this requires the headset to be frequently retightened (like everytime you go out?). Loctite is not a total answer!