....... You could tap a thread in your hanger to take a non weird rear mech.
Had a lot of thought about this, looked elsewhere. A very good read is this
https://www.redclovercomponents.com/blog/1 amongst other places.
It's a nightmare with these Simplex drop-out mounts because I think there are essentially three. The thread in the Simplex Competition would be easy to sort out, less easy is shaping a Campagnolo stop / flat, and perhaps the big issue is that the plate is not at all in the same plane as a normal drop-out, it a fair bit more outboard so there could be issues with sufficient derailleur travel.
The best modification would actually be a custom made B-stop plate and top-bolt out of AL for say a 9 speed 105 Shimano derailleur - all it would need is a longer thread in a different diameter. Otherwise, the second best would be to cut the lot off and get a normal hanger plate - it would look a mess and the frame would be molested for good
I've decided I won't go down this path, and have just accepted it's Simplex and four speed at the back all the way (I mean the down-tube gear lever boss demands it will be Simplex too). What I do know, is that at least, Simplex did make their future derailleurs compatible with older drop-outs.
For this bike, I would be happy enough to know I've not molested something historic and live with a good condition working but rather nasty looking Prestige that I know for sure will fit and work. Understandably, the prices of the Juy 543, Export 60 and 61 are utterly mental since they are about the last silver shiny refined Simplex mechs made - in small numbers too I would imagine.
There's no point of me even thinking about such a thing TBH. For such a price, I know it would be better value just to buy a complete bike with such a derailleur on it. Perhaps with patience something might turn up locally for peanuts