My 1976 Motobecane Grandtouring front fork broke just below the right front fork.
I would like to get my bike going again. Does anyone have any good information on what options I have for replacing it? How interchangeable are forks for old French bikes. What are the key measurements when spec'ng it out? Where are the best places to look to find forks. 1st priority is to get something that fits to get me back on the road, 2nd is to find a comparable red Motobecane replacement. Of course money and location are relevant...
Bummer. Finding a correct Moto fork for that bike will be quite a task; more or less depending on your frame size. You'll be doing well to find a period-correct crowned steel fork in the right length, that isn't bent. Be real careful about that. In the absence of a Moto fork, I'd think seriously about replacing it with a standard (not French) threaded fork. It will be a whole lot easier to do headset replacements, and the head cups are the same size.
If you just want to get the bike rolling asap, your best option, if you can tolerate the look, might be a Tange unicrown chro-mo fork, which you can get from Alfred E Bike and a bunch of other places for around $40. I think they might be black only. Dimension also makes chromed crown forks for cheap, but the steel and overall quality is a drop from the Tange forks IMO. Of course, a builder could make you a fork but there you're talking close to $200. Ebay is an option as well, if you're patient. Nice crowned threaded forks are getting harder to come by, and most of the decent ones I've seen recently were being sold out of Europe.
I'm not builder, but my guess is repairing your present fork is a long shot.
 I neglected to mention that if you switch to a standard fork thread, you'll also need a new stem.