Damn nice bike. If my 1966 was anywhere near that original, I wouldn't be doing it up as a cafe racer!
I notice the engine number is YB 16634, which looked really familiar to me, so I went out to the shed and looked at my spare crankcase, and the engine I'm using in my bike.
I have that one bracketed closely. The engine I'm running is YB16641, and the spare is YB16630.