I'm wondering what's the smallest possible fossil fuel _generator_ you could make, you could effectively make an ebike "hybrid" and it could be way more efficie

I'm wondering what's the smallest possible fossil fuel _generator_ you could make, you could effectively make an ebike "hybrid" and it could be way more efficient since you can just run the engine at optimum speed constantly. Off-the-shelf units are 1000W, but a 200W constant unit would be the best.