On the face of it is that plug is fouled with oil rather than dry carbon? If the head gasket is suspect, there is scope for poor compression and a leak of gasses into the crankcase which I would expect to exit the breather more than is usual.
While the head is off I'd be inclined to lift the barrel and check the bore, pistons and rings. Inlet valve guide condition on the poor cylinder is another source of too much oil in the combustion chamber. Exhaust gases may give a further clue, black smoke and smell of fuel is too rich mixture, blue smoke is burning oil.
Swarfy.
