Making a model boat with pop-pop motor has a difficulty, when you fasten the motor on the boat.
If the boat is quite an ideal construction, with minimal displacement, and optimal intersection, the boat can be unstable. So it is practical to make some outriggers, e.g. a trimaran.
First guess is to calculate the immersed volume from the weight. For instance the motor and burner weight is 170g, the weight of the whole boat is 230g, than you need a whole displacement with about 400 cm3.