The long cage is to accommodate larger gears needed for easier climbing. If you want to keep the Simplex Derailleur, I believe the jockey wheel can be replaced with Sun Tour jockey wheels. The most practical thing would be to replace the derailleur with a Sun Tour Cyclone or Vx
Would like to keep everything french, but which is cheaper and more reliable?
Guess what, cheaper and more reliable is not french. Japanese (Suntour) is your cheaper and more reliable option. There are nicer Simplex derailleurs (much more reliable than the cheap plastic one), but they cost quite a bit more than your mid grade Suntour derailleur would cost.