outssider pegged it; and of his three reccomendations, mounting the engine sideways is the least painful, in terms of additional cost/effort required.

I have a YS, and it doesn't care where the fuel tank is. But I was concerned about potential cooling problems here in the sub-tropics, and I saw the two guys at my field that had this model before me struggling with attaching/removing the glow igniter on that inverted head.

The older I get, the more I"m really into hassle-free things. So I mounted my YS 91 sideways. It looks okay, and the "square" mounting holes on the UCD make placement a no-brainer; just rotate the engine mount ninety degrees.