Here some pics of the Yamaha PCV valve that I fitted onto my Ariel single.
The whole set up fits without having to make irreversable changes to the enginecases.
Note that the crankcase venthole is only on later model singles (I think after '53 -?)
Text from website where the Yamaha reed valve is for sale :
Reed type Positive crankcase ventilation Valve. Valve hooks to engine top breather vent and allows expelled air out but reed prevents air re-entering quickly so a very small vacuum is created in the crankcase. A small power increase has been noted during dyno tests (most notable on pre 77 engines) and the crankcase vacuum helps prevent crankcase pressure from pushing oil past oil seals and case joints.[/i]http://www.mikesxs.net/product/15-0677.html