When the first Van Loo version of SIG's Chipmunk kit was still new, I built one for a G21-40. Van Loo had used that engine, but I kept trying to run it in a classic breaking run, and it kept "running away". I pulled the 40, and used a 46 instead, but then the fuel mileage was much worse, so I planned to cut into it to swap tanks (hadn't left the bottom part of the firewall off to allow the tank to slide in and out). I never modified it, and in fact never completed the entire finish (I test flew it in monotone, single color - metallic blue).
I swapped away the 40 for a backup 46, built a Skylark (always loved those), and the Chip became my hangar queen. Interestingly, I ran across someone running their G21-40 in a wet two stroke, and getting excellent performance, but one difference was fuel. I had been using Ritch's Brew 5 % Nitro, 22 % oil, partly castor, mostly synthetic. Whoever (and blame my age for forgetting any face or name to match) had that other 40 used SIG 5 % all castor fuel -- 25 % oil, I think. If I ever have another G21-40, I'll be using more oil in the fuel, and less of it will be synthetic.