Re: clip latches or repro Hartwell latches.
I bought a set off a chap called Mark, they were perfect replicas apart from the word "PRESS" was engraved instead of embossed as the original latch tongue was cast and the repro's are machined. He has sold out but he is thinking about making some more. They were £500 a set of 4.
Alternatively Gelscoe in England may have some but they may have bought them from Mark.
Here is a link to a filler cap on ebay from Mark and you will be able contact him that way. It is what I do. GT40 FUEL CAPS ORIGINAL CAST ALLOY CORRECT SM450 SERIAL | eBay