RE: Another Brillelli Masterpiece - Briggs 34cc Four Stroke
You can mount the Briggs inverted (I have it inverted on my 1/4 scale Fokker DVIII)
It takes straight gas.
A small mill/lathe combo would be perfect for making the conversion parts. (all you reallyl need is the prop spacer, the motor mount can be made with a bandsaw and a drill press. A mill would do a nicer job though.)
The conversion is really easy. A straight pipe would be perfect for the exhaust.