Oily exhaust port

Is the plug oily on that side too? My money would be on the usual suspects - inlet valve stem seals. Followed by worn piston rings.

Worn guides are unlikely to make the exhaust port oily. Badly fitted guides or a cracked head might - but you don't want to go there. If it were me, I'd pop the valves out, clean the head, give it a visual inspection, check all the oil drain channels are clear and reassemble with new stem seals. If the bores aren't badly worn but you don't know the history of the bike then, now it's apart, hone the barrel and chuck some piston rings at it.

Summer's coming, put it together and ride. If it doesn't smoke like a two-stroke then you can live with it.

If the pistons have a large slot through the oil scraper ring, I would replace them I did with mine, as I was told they were prone to breaking at that point

I can't remember what the original rings were like, think I found out about the pistons on the Norvil site.

Too late now but if the engine was still running the blue smoke on acceleration points to rings, blue smoke on over run the likely inlet valve. Mine was smoke as I left the lights seen by my fellow workers just before works entrance, as normal on the left side, so new rings and a hone to sort it.

Thinking back to those polished conrods and what a previous owner might have been up to ... when you put it back together and adjusted everything, check that the valve springs are not coil-bound at full lift now you've put the insulator washers in. To do this, rotate the engine so each valve in turn is at full lift then lever the valve down (with a screwdriver or similar) and look for another millimetre or two before the spring is fully compressed. More is fine on a road engine, less is something to worry about.

… if the p/o fitted Colsibro guides and corresponding valves, as suppled by (e.g.) BLR Engineering (http://blrengineering.co.uk).

 I've been running these in my 750 Fastback for the past 1000 miles and so far at least they are entirely satisfactory. 

Allan may find the attached pix useful: the Colsibro guides and associated valves in my bike.