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.