Wing Ding I would not go with a gas engine on this model. The weight of the mag and or gas engine is going to take away from this models performance. A cap is an aerobatic plane and the 25 cc homie gas engine will not deliver the power to weight ratio needed. If you do install the 25 cc homie please convert it to glow (alcohol burning) and remove the ignition. You will get more power and it will run cooler and weigh less.
The other alternative is to follow the manufacturers suggestions on the recommended engine sizes. Believe them, they want the plane to perform well.