BME has a .90 Supertigre engine that has been converted to spark ignition for gasoline use. It also comes with a special gasoline carburettor and needle bearing crank/con rod which allows for a 32:1 fuel/oil mix. At 15 cc, it is probably the smallest gas engine commercially available, although you can convert any glow engine, even a .40, to spark ignition with an aftermarket electronic ignition module, however, without needle bearing con rods, you will still need to run a really oily fuel, something like 10:1.
http://www.bmeengine.com/point90.htm

That might be a good combo with a 1/4 scale Cub if you absolutely hate the idea of glow engines.