I'd use a Ryobi 31cc gasser on electronic ignition. Or if you want it totally nuts-- use a Poulan 42cc on electronic ignition.

Both can be had for pennies compared to the Zenoah or any other commercially made gasser. Plenty of people around this board that can machine the parts for you and sell you the ignition system.

I built a 54cc Poulan engine for $230 complete with synchro-spark ignition. It is swinging a Zinger 22-8 at about 6900RPM. You can probably build a Ryobi for under $175. The ignition will cost you the most. But it's worth it. It will save you weight and make it easier to start.