Suntour Vx is a great choice, the one that fits to a non-integral hanger, like the one on this old Jeunet...



This is what the Suntour hanger will look like, and the derailleur hung to it is a Suntour, also, and a step or two lower than a Vx...



Cage length depends on your freewheel cog sizes.

If you need one, I have two or three tucked away. Let me know and I will post on the For Sale forum.