Compliments, Norman, for remembering my idea, I drilled from chain case into the gearbox filler tube, just to ventilate the primary case via the gearbox vent. It should do I hope, without filling up the gearbox with primary oil, maybe I fit sort of shield around that drilling, but I will try without this first. Certainly you´d have to get the engine breathing right before thinking about that idea of chain case venting. For this I packed a real seal between the drive side main bearings, no scroll any more. A steel ball type one way valve seems too massive to me for engine breather, better get a reed type valve - but check it for minimal contacts around the reed else it will stick a lot with any oil film in there. I did some with a feeler gauge blade and placed at the end of a hose high up like under the seat. So any minmal oil up there will drain down before exiting to free air through the breather - worked on a few bikes like this without oil mess and catch canister. The last idea was to do a phenolic paper one way valve screwed onto a valve spring cap to breathe into the oil tank (UFM ) . As said, you´d look for an engine without lots of air leaks so there is not a lot of air that has to exit via the breather, just a bit from blow-by from pistons.
Vic
Reed/disc valve onto valve spring cap:
hose end reed valve: