The AGC 300 brakes aren't practical since they're ultra short reach and won't work on an old Motobecane.
The easiest option for the OP if he has a fairly tall frame (56cm or so plus) he can cross the rear housing loop across to the other side of the seat tube and on to the brake. Or, if the frame will accept it, he can mount the rear brake to the front of the bridge which will put everything on the right side.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.