You'll likely vibrate 1/2 of the glue joints loose with that engine on that airframe within the first few flights. For the 26, you need something in the neighborhood of 8-10 lbs airplane. I have a G23 on an 80 inch 13.5 lb airplane and it's a great match. I've seen the ST 90's converted to gas, you might try that route too. Or the G-20 ei is a good choice, but a bit heavy for the power I think.
There are gas options (few, but some) for that airframe, but a 26 cc isn't really a good one.