Originally Posted by
CV-6
Simplex LJ and SLJ would be period correct but pricey. Suntour V-Luxe shifts well but would require notching the derailleur hanger for it to work. I cannot tell from your photo if you have a hanger or claw so that may not be an issue for you.
Looks like a claw. Easy to find a Suntour or Shimano which will work. A Huret/Sachs-Huret could also be an option if you wanted to stay all Frenchy.