That's the problem. Using a gas engine instead of a glow will always increase the weight. Your ony offset is in the amount of fue rquired for a flight. TYhe gas engine will use 1/2 to 2/3 the amount of fuel so you can run a smaller tank. Unfortunately the ignition, ignition battery, and the mass of the engine frequently offset, and then some, the weight of the reduced fuel load. You have to decide if the plane can handle the weight increase and still fly as well as it should. Not to many planes designed around using a 50cc engine that weigh less than 15.5 pounds, and generally a bit more.