Regarding parts in the UK I would try
http://www.trialendurodirect.com/ which is the online shop of Gasgas UK if I have understood matters correctly.
I suggest you buy the piston first and then get the cylinder repaired to match the piston.
Regarding frame number I don't know.
If you suspect dirt has entered the intake I would also consider checking the main and conrod big end bearings. Not fun to rebuild the top end just to learn as soon as you start it up that the bottom end bearings are shot too.
Do you have any idea on how many hours the engine has ran since last rebuild?
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