RE: 50CC Gas for 1/3 Spacewalker
I have a 20 year old Walker Sachs 4.2, that I'm putting in mine. I doubt that it'll ever go over half throttle.
If I were to buy a motor for the big SW, I'd buy a Zenoah G-45 or G-62, keeping the mag. You just can't beat Zenoah's track record for quality and reliability.