According to the basic specs, the goldberg kit has an extra 4" of wingspan. I am going to start a GP 60 build this winter, and am going to use a weedeater 25cc engine, if I can get enough performance out of it. It is a light engine (a bit heavier than the G20) and I have ported it a bit. Right now, waiting on a new ring, hopefully it helps, cause the original ring was worn out.
It would be safe to say the goldberg kit would need a strong 25cc gas engine for good performance.