Well that Cox Tee Dee .010 would be tricky to convert to gasoline. The 1/4x32 spark plug is larger than the engine!

Then what to do about the CDI ignition unit and such too.

But with electrics taking over RC airplanes more and more, manufacturing a engine smaller than a .40 might be a problem. Is there a market for smaller gas engines left out there anymore? .25 gas engine for example. It isn't like there are a lot of model airplane kits or ARF kits out there for that size nowadays.