I'll have to respectfully disagree with Stephen on the idea that gas engines aren't good for 3D until you get to the 50cc range. That used to be the case, but nowadays there are many 36-40cc gas engines that perform very well in 3D applications. They are getting lighter and more powerful as competition increases in the small gas engine market. Companies like Brillelli and BCMA are making a big impact with their lightweight, purpose-built airplane engines.

I have a Goldberg Extra 330 like yours and I put a BCMA SPE 40cc in it after many unsuccessful attempts at getting a Supertigre G3250 to run well. The 40cc pulled it out of a hover with authority after it was broken-in, (about 2-3 gallons of airtime). Hovering was no problem at all, and taking off straight up while doing aileron rolls would continue as far as I could see the plane.

I think that the Saito 220 would be a great match as well, but the BCMA weighs only 9 oz. more with ignition, etc. and you can carry a lot less fuel to make up for some of the weight gain. And, the fuel is a lot cheaper.

-Pat