At present the NV-Engines GX40 gas engine is the smallest production gasoline engine. It uses a OS P5 glow plug. It works with any of the gasoline mixes out there. The engine is a .40 cubic inch displacement size or in metric 6.5cc. NGH makes the 9cc gas engine but it is spark ignition. I never tried it, but it make work OK with the OS G5 glow plug too. There is a 3.5cc RC car gasoline engine out there someplace. One of the RC car companies makes the engine and sells it with some RC cars. It uses spark ignition though. There may be a RC car .18 engine out there too I remember reading about it somewhere.

The regular glow plugs do not work as a catalyst for ethanol. But if you use methanol the glow plugs work fine. There is a minimal amount of methanol needed in the fuel too.Some guys in the past used to mix gasoline with glow fuel, mostly as a experiment, but it did work Ok. You could add methanol to gasoline too. Some guys did that as well. They called the special mix "Glow Gas" fuel.

Now there is a M85 gasoline out there. It is used mostly in China, but there are some gas stations in the USA that sell it. I think they are mostly located in Washingon state and Oregon though. M85 fuel uses methanol instead of ethanol.

Also for glow plugs, the OS P5 ultra hot turbo plug works with gasoline. It doesn't matter what type either. Also OS just came out with another G5 glow plug that works with gasoline too.
I know they use the P5 plug with the NV-Engines GX40 gas engine. You use Klotz Benol oil with it. OS is selling their 10cc gas engine that uses the G5 glow plug. Also the ultra hot P5 glow plug was originally used in RC cars, so there may be some other competitor ultra hot heat range glow plugs that may work with gasoline too.