Got the spare engine today. It is a later model one, as I thought, but most of the parts are identical. I only got it for the cylinder but it seems in great condition (being 30 years old) I am so tempted to just remove the old engine, bolt this in and try and start it. Great compression and no slap. Looking though the plug and exhaust holes, the piston, rings and bore look like new.
It has already had a couple of squirts of plusgas so far on each bolt and thread but leaves me with a bit of a dilemma. Do I; run this engine knowing that the engine number is wrong for the bike; transplant the top end onto my original engine or take off the right crankcase from my original and use that on the later engine so the engine number is correct (assuming that it fits)?