Years ago, I did a number of conversions of Honda GX 4 stroke engines (still running today) and although they are heavy and under powered by most standards they work REALLY well with planes like the WWI 1/4 scales and the Sig Rascal 110. I am not up on what is available today, and I am sure the era of gas yard tools is ending, so what if anything is out there now able to spin a 20-22 inch prop at 8-10k RPM, enough to fly the balsa USA 1/3 Cub.

I'm zero help to you with conversions. But you can buy some nice twin gassers for not a ton of cash. Valley View RC has a 50cc twin for $420 and a 60cc twin for $439.
These are nice units and they are great people to deal with.

David

Those are 2 strokes, it isn't about the money so much as it is the 4 stroke sound and suitability. That said, when there were no 30-40cc 4 stroke model airplane specific gassers the Hondas and Stihls were available, then Fuji sold a less capable 4 stroke engine for 3~4x the price of the Hondas so why pay more for less? But the goal is a 4 stroke gasser.

I understand. I hope your search is fruitful.

David

Look into a Stihl 4 mix. Its a 4 stroke that uses pre mix fuel but is a 4 stroke. Im converting a 27cc and will be running a 17" or so, but this guy runs the 32cc with a 20" in a 1/4 bipe . Look at his other videos,he's converted a 64cc as well as an 80. They are quality engines and last forever.
Maybe the 32 will do it for ya?

Staton has four strokes 16. All Engines Two and Four Cycle All Sizes