Given all the bad press they get in this forum, the Simplex derailleurs are actually quite good for their day and their price range. Yeah, you can always upgrade to a SunTour V-GT/Compe-V combination, as it was the standard upgrade back in '73, but you're taking away a major bit of the French-ness of the bike. Biggest worry you'll have is a cracking on the front derailleur body, usually avoided by not removing and replacing the derailleur if it doesn't have to be. If they're working, leave them be.

That bike is just in too nice a condition to be modifying. You've got a nice little time capsule there. I look at that orange and think back to the insane extent I went to on a Roger Riviere to restore it to that condition . . . . . and then discovered that it was a POS compared to the Peugeot when it came to riding.

As redneckwes put it, that's a time capsule. And time capsules are far and few between when it comes to the bottom of the line bikes.