Nathan the original tyres were 350 x 18 Cocker Tyres with a diamond pattern. I think they are still available but with shipping an arm and a leg.

I bought 350 x 18 Cheng Shin for mine (Same tread pattern as the Dunlop K70.) I’ve used Cheng Shin on other bikes and find that they wear much better than the Dunlop’s.

I originally fitted 400 x 18 Cockers, but they are too fat and rubbing at the rear.

You might have to search, but I bought mine from an on-line firm called “ Round and Black” Ron

email (option): ronpier@talk21.com