graham500, Thanks I correct it.

psgugrad, Yes the main limiting factor is the weight of the ignition system and battery to power it with.
It is possible to build a .049 gasoline spark ignition engine. But the CDI unit can only get so small. One could use a smaller battery pack to power it with though. Or a single battery pack with a iBEC to power the ignition unit with. So maybe something along the lines of a 1/2a Q-Tee plane or similar might be doable as a gasoline spark ignition engine powered RC plane. I doubt a high performance plane and or engine combination would work as a gasoline 1/2a engine is not going to have a lot of RPMs to it. But maybe I am wrong. I haven't gotten around to making one yet myself. I do have some really small spark plugs though. I have been thinking about it. I did get some micro-size spark plugs too. If Iremeber someone did make a gasoline spark ignition .12 engine many years ago and they did sell pretty well. So maybe a bigger 1/2a plane with a .09 to .12 engine would be the way to go. Since we now have 1/4x32 spark plugs and they have 1/4x32 glow plug heads for 1/2a engines it is easier to make one a spark ignition engine nowadays.

Dale Deitrich makes these little 8x40 size threaded micro spark plugs. Here is a pic of them next to a couple of 1/4x32 size spark plugs.