Hi Nick,
Allbeit that you have established there is some wear in the bores, I doubt that is the main reason for the sudden oil burning and discharge from the breather. At this stage, it would be worth checking out the timed breather (in the timing chest). It is a cast aluminium disc that runs against the inside of the outrigger plate. The aluminium casting has 2 small lugs that engage with the face of the inlet cam, and they are prone to wearing to the point of not being driven by the cam.
On re-assembling the engine, make sure the crankcase through bolts are correctly tightened, starting from the left hand side (they also secure the primary chaincase). Then progressively check that everything in the timing chest is securely tightened before fitting the timing cover. It has been known for owners to think all is well when just the timing cover nuts are tight - but that leaves the cam followers and outriger plate loose enough to wriggle around on the through bolts until (usually) the oil feed post snaps off! The extra stress also often results in a cracked timing cover.
Regards,
Eddie.