I've played with these engines a little and figured out part of why they have low output. The exhaust port timing is very low, I've measured close to 110 degrees on some, that duration needs to be near 150 degrees to get decent output. The ignition method will not change the engine output but getting more air and fuel in the combustion chamber will (combination of increasing port timing and slightly larger carburetor)

Has i promised you

Here it goes some pictures of my converted Poulan Featherlite 21cc

With this Graupner propeller it swings @ 6000 rpm´s (15/8 propeller)
It wasn´t a new engine.

I just changed the Bearing of the crankcase !!!

Yes of course the it will need of timming port changing , a different muffler , and of course a different carburator

But as is , i think is capable of lifting a trainer configuration

Perhaps with a 14/8 or a 14/10 more rpm´s i think , some day i will try it just for fun

Opinions are welcome

Best Regards

Nuno Monteiro from Oporto Portugal