It sounds like you've already verified it's Swiss, and of course it's easy to tell as the fixed cup would be left-threaded. Motobecane did both French and Swiss at various times, so you'll want to be sure.

Either way, replacing the bb will be expensive and/or difficult, so just use your existing adjustable bb. The crank that's on there is probably just fine; in a lot of cases, people "upgrade" from well made stock cranks to inferior replacements. Check the chainline using the existing double, and consider that you may be able to move one or both of the chainrings to the other side of the spider as well. If you can't get your chainline sorted out that way, you can use spacers for the chainring or try a different spindle length, though spindles are getting harder to source.