I'm not impressed with the advice you've gotten so far.

French bikes use metric-size tubing. Your seat tube is 28mm instead of the standard 28.6mm. A non-French derailer that's made for a 28.6mm tube may tend to slip. It doesn't always happen, but when it does it's a real PITA. Your UO-8 is set up for a front derailer with a housing stop and those are hard to find these days. A Simplex SJA 102 like mine solves both problems, but don't ask me where to find one.

Note how the cable housing runs to a stop on the derailer, just like on your Simplex Prestige. You can rig up something that will work, but this is the best way.