NV-Engines sells the GX-40 gas engine, it is 6.5cc or .40 displacement and uses a special glow plug that works with gasoline.

There are a couple of 3.5cc ( .21 ) sized RC car engines, they may be larger in displacement though, but they have RC engines for the 1/8 size cars now. I forget the brands or information about them at this time.

NGH has a 9cc gas engine that is being sold. It uses Spark ignition. They have a 17cc engine as well.

Horizon has a 10cc Evolution gas engine and a 15cc one too. OS has one in that size range now and it comes in two versions a glow plug and a spark ignition version.

Saito has a 14cc four stroke gas engine and a 11cc gas engine too. They are spark ignition.