The way I see it, casting/machining a block is not that hard an endeavor- I have machined single cylinder engine blocks to fit an existing rotating assembly with nothing more than my cheap little import lathe, mill and a boring head. It can be done- plan to spend ALOT of time standing there making chips. The problem lies in the crankshaft for whatever you are building- getting a double throw crank put together that will stand up to the situation is much harder than the casing to put it in. Most of the "model engine" sites that give any details on how they make cranks are geared towards the folks who make multi-cylinder model engines just to run them on a stand- not put it in a plane and turn it 8,000 rpm.

If I could figure out how to accomplish this, I'd build me up a twin based on Echo cylinders/pistons rods.
Anyone got any ideas?