Well, in my experience anything over 10-11 lbs (with engine) would be pushing it for a 26cc Gas, unless it was on a tuned pipe, and even then not too much more for the 'overpowered' feel you are looking for. And I am defining 'overpowered' as enough to fly the plane with 3D aggression.

Between 11-14 lbs my best guess would be the 35-45cc range, and between 15-18lbs start looking at 50-60cc for that kind of power.

Now you need to research the engines available in the weight class you are anticipating, and go from there. Remember to watch your wing loading!!!