It seems to be either feast or famine with vintage 18" tyres. Pirelli have launched a new line of 18" racing rubber for vintage race bikes.
It's the spec tire for CRMC, Classic TT and Classic Europe Endurance Cup. Those all used to be AVON.
I believe its the same SC1 and SC2 compounds that come on the Diablo Supercorsa.
Anyway another option - https://www.pirelli.com/tyres/en-ww/motorcycle/catalogue/product/phantom-sportscomp-rs