Thanks for the input folks, I fully agree that valve seals/piston rings may be the problem (the smoke is getting darker) but whether this or even a gasket failure the cost of investigating and solving requires more cash and time than I have.
Rebuild wasn't guaranteed, I bought the car for a grand and as it had 17" teledials with good tyres, a decent tan interior, aero spoiler and a new exhaust etc. I reckoned it was worth the gamble.
I'm inclined to swap some of the parts to my other twinnie, the question then is whether to sell on as a project or strip for parts and scrap. The latter option makes the most sense economically but seems a shame.