Getting more horsepower from a Briggs & Stratton?

I have fabricated a stump grinder using an old 5hp B&S lawnmower engine. It grinds stumps, but it grinds them slowly. If I get aggressive with the wood removal, the engine starts to bog down. I'd like to get more power from the engine. What would be some modifications that I could do. I have a fairly well stocked garage (welders, grinders, drills, etc).