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.

Step 1: Main Hull

The main hull is made stronger for the motor and the burner with the Baselint Method, from laminated wood planks, and fastened with epoxy resin adhesive. It has a windshield too. But the shown balancig method can be made with any other type of main hull, wich has no stability alone, and has to be constructed as a trimaran.

Step 2: Outrigger Hulls

For insnance the outrigger has simple geometry, the sides are rounded, and the bottom is cut down in an angle at the bow and stern. It is from cardboard and is painted waterproof. The balancing method is good for separate outriggers, where the outriggers are hanged up with strings from a stick, and are swimming, like buoys. When you make the balancing with fixed outriggers (like on the picture) you can fasten the sticks with spacers on the board for measuring the level.

With separate outriggers you can determine the centre of displacement (here is the outrigger horizontal), and take the string to this point.

Step 3: Balancing Bracket

The balancing can be made with a bracket. This is fastened with rubber bands on the main hull (on the picture the two angular lines in the upper frame). The height of the frame shall be greater, than the main hull, the difference gives the possibility of adjustment. The distance of bracket from the board can be set by spacers (the prism in the middle of the upper frame). The long stick on the picture is fastened under the two outriggers with string-loopings in the centre of displacement. The underwater stick can be set with the spacers up-down, and moves the outrigger hulls upwards or downwards from the board. You shall take care of other stability rules across, and along the main hull. E.g. cross stability is better with greater distance from the main hull, the longitudinal stability is better with greater distance between main hull and outrigger displacement centre. The water-lines shall be set horizontal.

So when the center of buoyancy is on a vertical with the center of gravity, tle outrigger hulls are horizontal. The loopings can be set near to, or far from the main hull (but in equal distance), to let the main hull cross section be horizontal, without side slope (put the motor gravity centre in the axis of the main hull). The loopings can be set on the outrigger hulls forward and backward too, to let the outrigger hull waterlines horizontal. A separate stick (main picture) is on the hulls, and shows the height difference between the main hull and outrigger hull levels. When the waterlines are horizontal, and the weights and hull-levels well-divided, you can measure the positions of the main hull and outriggers.