Hi Stuart.
i owned a Sportomatic 911 back in the 1980’s. It was a ingenious idea where you had a normal manual gear stick, but only two pedals! The clutch in the Sportomatic gearbox disengaged when you touched the gear lever. I used my 911 as a daily driver into London. A lot of 911 owners were converting their Sportomatics gearboxes to manual cars. So there should be a lot of spare Sportomatic gearboxes around. I think they fitted the 911’s with them up to the early eighties. So you should certainly be able to get your converted if you can find the parts and a company who knows about Sportomatic gearbox’s!
Hope this is some help.
At least your car would be saleable when you want to sell it, as it will still be 100% Porsche!