Congratulations on your new to you, Seven! Hayes Harris has a good reputation which helps when purchasing a Seven.
The AC oil pressure gauges do show up several times per year on eBay UK. Here's one now, though you'll need to ask the seller if they will consider shipping outside of the UK. It's higher priced than most go for, so you may just want to wait or have yours refurbished.
https://www.ebay.co.uk/itm/185682975276?hash=item2b3b915a2c:g:gPEAAOSwCTJjVWuL&amdata=enc%3AAQAHAAAAsBqnibmHlOqXoHjB4s2bzqa5uhzYY49E7UDrK%2BELTpFLSwcp5rnAHGRUWEBFI%2FA%2FrE3dBmGwLyiK%2BngW%2FlCSZorclu%2B3lK%2F3IN2rVxhE2FGQjnk%2BCfUl8JRucESbFk0aVwYnTA4zQ7E%2FTvUcbB00R58RorM8uqG3rGahh1qNFqBV6j%2B3FAtgsMiCZ6DagMCHvOdm8iua%2BWkoDIsphipaQe6dzhbbyepGGasSaLuGazWJ|tkp%3ABk9SR97Qs_atYQ
Best of luck!
Edit to add: Looking at the pictures of your car, I think you meant to say your Temperature Gauge is not original. That was not an uncommon swap (to Stewart Warner) - the AC Temp Gauges are very rare. You may want to check with Redline Components in the UK as they made a (now sold out) limited run of reproduction AC Temp Gauges. I've asked if they would be making another run of them, but have had great difficulty in getting a response...I've been asking for two gauges.