I had looked at placing a evolution 26cc in the Venus II sometime ago, but it did not look like a good match.... too heavy of a setup!.... You woul come out at around 10lbs!
The G20 may work just fine.... Some have said that the zenoah 20 will pull the same as the g28? not sure if this is true or not...
not sure of the weights though....
In my experience, I had a evolution 26cc with a tuned pipe on a Funtana 100x and it was too heavy!!
I would say that you will probly be dissapointed with anything in this plane other than Nitro Engines!!

the gooy stuff on the plane and a great flier is much better than a Clean plane and a horrible or sub-par flying plane....
IMHO
Justin