Decades ago a friend of mine who had a small metalworking lathe in his garage, was using it to make small gasoline engines - model airplane engines, like yo find in hobby shops. He was making pistons and cylinders that were polished down to the 10,000th of an inch. There was no blow-by. There must be a method to make these drones gasoline engine powered. They could stay aloft a lot longer, and you would hear them coming.