Both two and 4 stroke pistons can be moved. Std clearance is around 0.035~0.04mm on the NSR's, I'm sure the service limit is somewhere on site but I can't find it, I'd guess around 0.05mm, I've seen Matt advise 0.045 clearance on racebikes to avoid nipping up on long straights.

I never clean my cylinder heads or "touch" my pistons if I can avoid it

If it runs fine and the top of the piston looks like the perfect one here http://www.apexleisure.co.uk/pistondiagnosis.htm I'd just run it and enjoy it.
Re ring it every couple of years by all means as ringwear is high on all two strokes and clean the powervalves etc. but if the jetting is right they're no less reliable than anything else.

All of us idiots only seize them up so much as we keep jetting them too lean.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread