I believe that pistons for the Wilf Green conversion are very hard / nigh on impossible to get hold of.

IIRC the Wilf Green 300s used a Mahle piston that was used in a much overbored 250 cylinder & only came in one overbore size. Piston life in the ETZ250/251 seems (anecdotally) to be long, so maybe not so much of an issue - unless you have a problem (lube failure or other seizure).

No idea of availabilty - guess it depends in part if it was a "standard" Mahle piston or a "special" made for WG.

Nigel B.