The club’s thermostat reference gives 059121119 for the O-ring. When ordering for a Mk2, check the engine and housing arrangement as well as that number, particularly on a modified car. Also check whether the chosen thermostat already includes its seal, so the parts list does not leave it out or order it twice.
CarParts247, which I help with, lists FEBI 17966 (https://carparts247.co.uk/febi-bilstein-162225.html) against 059121119, with a 50 mm inside diameter and 4 mm thickness. The listing contains one thermostat seal, not the thermostat or housing. Use those details to compare the required O-ring, and inspect the flange and sealing surface if the old assembly has been leaking.