It's been a very long time since I had to deal with this sort of bikes so if I'm wrong hopefully someone will correct me but what I think I recall is that the Lark rear derailleur should be compatible but the French used a derailleur hanger which must be removed then have a newer one brazed on which is more work then it's worth; and the front will not due work due to the clamp being of different size (French) vs the English and the clamp will not tighten properly, optionally you could have the front brazed on too, but I wouldn't do any brazing for any derailleur.

You could go to E-bay and find a better quality NIB or NOS Simplex Prestige but not the delrin version. One like this would work fine: Vintage Simplex S001 T Prestige Rear Derailleur Friction Short 70s | eBay Of course it might cost $60 but there would be no need for brazing a derailleur hanger so it would probably cost the same. Go on E-bay and search Simplex front derailleur, make sure you measure the clamp size you now have and get the same size if you need a new front derailleur, these cost about $50. If the Shimano shifters are the band type like yours is it should work, if not again E-bay has these for around $25. Optionally you could go for barend shifters but those will cost more. Also if you find a Huret derailleur for the rear that would work too because they have the hanger, and it's a better derailleur than anything Simplex came out with. But if you go Huret I would go with Huret front and shifters too just so the bike has the same thing.

If I had your bike and the rear derailleur is bad I would find primarily a Huret or secondarily another Simplex that is another model up from the plastic body one. I use to have a Puch bike with that same derailleur you had and it shifted so poorly, no matter I did or a bike shop tried to do it almost made me give up cycling back in the early 70's thinking all rear shifters were that bad! but I was young and limited in my knowledge of such things and didn't realize there were better derailleurs on the market.

If you like the bike, which it sounds like you do, then I would spend the money to upgrade it regardless if the bike itself is worth it. I was reading somewhere that those frames actually rode nice even though they were heavy, so you may have a nice riding bike you just need to improve the mechanicals enough to make it enjoyable all the way around without breaking the bank and $150 or so for derailleurs and shifters is cheap but you would realize a big improvement in it''s operation especially if you go with Huret.