Another smoking curvy

Bike has been standing in a shed for a while, but has been started up fairly regularly.

Srarted it yesterday, and it ticks over fine for a while, then starts producing loads of smoke. More you rev it, the more it smokes.

Let it cool then restarted - same effect.

Rear plug has oil on it (front is OK), and rear cylinder exhaust pipe is dripping oil, so seems every sign of burning oil which leaves me thinking either valve guides or piston rings. Airbox looks clean enough, so doesn't look like oil from there, and no sign of oil in carb.

I know enough to strip the head / barrell off, but would leaking valve seals / rings be obvious once you had it in bits, or is there another possible cause i've missed?

[ppbayeta]

Maybe the head gasket blows between the barrel and an oil feed pass. Or just change plug and try it

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what colour of the smoke is it? is it really thick?
it could be just water condansation if it stood for a while and didnt get a decent ride.