Parts were pretty easy to find when I built a 650 in the late 80s. I never got to play with a pre-unit so I don't know about availability there. But Norton pieces were few and far between. I gave up on a Commando motor due to the cost of having to buy a whole bike to get a few internal replacements. Of course I was living in Alabama and there was no internet at the time... may be easier to go Norton now with a much wider search net.
+1 on vertically split cases being stronger... and leakier.
