Okay. Fact one: Motos tend to be Swiss thread in the bottom bracket, not French. This means the fixed cup is reverse threaded. There thread pitch and diameter are the same as French. Sometimes (not always), you can thread in an English BB. They're different, but not as much as folks sometimes think (myself sometimes included). Try it if you have to (it may not work) but:

Fact two: most Motos were equipped with Japanese BB cups (Ie Sugino or Sakae) and replacement spindles are far easier to find than on other French makes; any old Japanese spindle will do. The walls of the cups are thicker on Japanese cups than most old Euro/French ones, so you cannot swap spindles between the two. Generally, old Japanese cups are marked somewhere with the maker's name, but I have an unmarked pair of swiss ones somewhere...

So a new crank may not be that aggravating, frankly. If you can find a Japanese crank and spindle, you're off to the races. Posting what the bike has on it (or pics) may be very helpful here. With an old moto, you would do better to trust the collective knowledge of the C&V board than here (although you'll get torched a bit for turning an old bike into a fixie, if you haven't done anything dumb-- like hack off all the braze ons with a dremel or rattlecan a perfectly good original paint job) you'll likely come out of the experience both intact and wiser.

Shops tend to be complete doofuses when dealing with old Frenchies, you'd have better luck bringing in a flying saucer for repair than an old Peugeot to your average bike shop.