I barely use the stuff anywhere if I can, usually just where the crankcases split.The new flame ring head gasket must be installed WITHOUT GASKET CEMENT.
Unfortunately I've bought something of a basket case G15 CS (actually an N15 but it will be a matchless when I finish) that has been severely plundered! Only thing left of the engine is some crankcases (118), cam shaft and cogs. I have however rebuilt a complete Honda starting from just a frame so I am confident given a few years it can be done, kind of a hobby where I buy bits in at a time and sort them out one by one. It also means you really learn the bike back to front inside out.The G15 engine does not use a G12 chaincase, but one made for the G15 models, that was adapted from the G12. The later models also had an added stud to stop the chaincase moving around. Buy a complete bike, as putting one together from parts is fraught with allot of difficulties.
I think from everything you've said I can go straight to getting a combat 750 cylinder and head? Is this the same with the crank? I'm not sure a Nourish crank is not overboard for me.
As for camshafts, I will probably want to stick to the Atlas camshaft, I've read the Combat cam shafts are more aggressive leading to the crankshaft flexing, helped by the bearing like you say but I'd like it to be super reliable at the expense of some performance, 50hp is more than enough for me.