Hello John
I converted mine last summer (according the discribtion in “Technicalities”) and run it without any non-return valve.
Crankcase pressure doesn’t seem to be a problem as it consumes a normal amount of oil.
I run on “Agib Speed” two stroke oil, and add 1% oil in the petrol, as the oilpump is adjusted to “next to nothing”, braces and belt like you say, (I think it was build to handle thicker oil).
I wonder if the bronce spacer acts as some kind of non-return valve.
Best wishes
Jan Buchwald