If the frame is indeed Swiss-threaded (and reputedly at least some Motobecanes of that era were), you may be out of luck in finding a new bottom bracket at all - particularly if you're looking for a modern external-cup version. My understanding is that that BB standard is now completely obsolete and was also rare even 30-40 years ago. But perhaps someone with more knowledge than I on this forum can point you at a source.

Sunlite does make a "repair bottom bracket" that is threadless and intended to be used in stripped bottom bracket shells (Velo Orange used to carry one, but I think they've quit doing so). But I don't think Sunlite makes it anything but JIS square taper. And your selection of spindle lengths might be limited as well.