It depends very much on what you want!
Genuine Kawasaki parts from the usual suspect; ZPower, Cradley Kawasaki, Z1 enterprises etc.
Aftermarket repro from; Saisei, Zpower, Z1 Enterprises, Legend Motorcycles, M&P Swansea, Keyster etc.
Aftermarket tuning; Debbens, Grumpy 1260, Dyna UK, Talon, etc.
Second hand: John Browse etc.
Specifically; Cant help with pistons, Kawasaki gaskets very expensive and i think only available separately for yours, valve stem seals in packaged gasket sets 'Vesra' etc seem fine to me, head gasket dont know either but many people with 'early' bikes wont use anything other than Kawasaki genuine gaskets, pistons, valve stem seals and so on.
Mine is a 'later' bike and therefore i have limited choices so; Cometic, Wiseco, APE, DYNA, EBC, and so forth.
Minefield was the word i was looking for
AL