I would just make one, or if there's room, run without one and just use some ultrablack silicone...

at any auto parts store they should have rolls of gasket material, get something thin like 1/64" or around there. to fit it to the engine, lay the material against the block side of the surface (usually easier than the jug side) and trace it out with an exacto knife on the inside and outside and trim for the bolt holes. the roll of gasket material should cost around what a new gasket would cost if you could even find one.

that's what I would do anyway, plus you now have a lifetime supply of gaskets for carbuerators and other parts on these things...