I think the main limiting factor is the CDI unit can only be made so small so far. The ignition coil is the main limiting factor as to size and how small one can make the engine.  I did have trouble getting all the stuff shoehorned in to fit in a couple of planes using the 9cc gas engine. With my Escapade .40, I had to cram the CDI unit in under the engine on a shelf. So as the planes get smaller with smaller engines it would only become more and more tricky doing it all.

But I can see a .35 or .36 size gas engine being made though. But the problem is whether the electrics have encroached on that engine size and plane size too much yet ot not.  I would think it is still a useable size in this displacement range.

So I think .049 gas engines isn't going to be coming to the flying field anytime soon. The CDI unit being the main problem.

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