Take a look at the Syssa Sap 180, which is a 30cc gas engine made in the USA. One of the guys in my club has this engine in his ESM spit and it fits very nicely inside the cowl (except of course the engine head and the exhaust tubes extend out the bottom of the cowl) It flies this plane very well. This engine has a rear carb and a rear exhaust and muffler that come standard on the engine. It costs more than the typical Chinese gas engine but since it has the muffler for a nice in cowl already with the engine it fits nicely into the plane and you don't need to spend $90 to $100 on a wrap around muffler to get the muffler to fit inside the cowl.

After seeing how the ESM spit flies and looks with this engine the Syssa 180 to me is THE engine to buy for this plane or for the ESM mustang. On the other hand I don't know what this engine would cost you in Australia if you can get it there.

Ed