I just checked out Ebay and there's a lot of Simplex components there. You might find direct replacements for what you have now, or you may choose components without the plastic delron bits. I think a lot of derailleurs will work so long as the FD fits the bike's down tube diameter and, if it doesn't used a hanger, the RD is compatible with the rear drop out.

Simplex Group with Shifters SX410 Rear Derailleur and Matching Front | eBay