Ok,good news and bad news!
Good new: he can source one from his supplier and they have it in stock!
Bad news:It's £46 plus postage,so just under £50...at that price i'd personally look for a new motor as Foxy said it's not a brilliant engine and you could get a better replacement for £70-£80.
However should you decide to get the new piston and liner,let me know and i'll give you the info.