Originally Posted by
JohnDThompson
Some things to keep in mind: Motobecanes of that era used Swiss-thread bottom brackets, so you'll probably need to keep the present cups and just replace the spindle with one suitable for a fixed gear chainline. Either that or use a threadless cartridge bottom bracket.
I'm not the Moto expert here, but based on a limited experience I'd say you might even have a French threaded BB rather than Swiss...that's what I've noticed with Motos and other FR bikes with those Nervar 3-pin cranks. As everybody says: the fork is the most valuable thing in the pics, and only of value to somebody with a different frame needing a 531 fork of that size.