The idea of racing lawn mowers has been around long enough that entire leagues in countries all across the world are dedicated to it. These leagues are overseen by governing bodies and even have sanctioned races. Getting involved in the leagues requires that you have a modified lawn mower. While the engine is essentially a simplified car engine, there are problems that are specific to lawn mowers and should be individually addressed. Most of the parts used for modification are homemade and fabricated.
Replace the flywheel. The stock flywheel is heavy and made of cast iron. While cheap, cast iron is prone to fracture at high speeds. Billet aluminum flywheels are available for sale and won't break down under stress. As an added bonus, they are lightweight and thus make your mower faster by increasing engine response and RPM.
Balance the crankshaft. Much like the flywheel, crankshafts are high stress areas on the engine. The original motor was built for low speeds. High speed applications, however, require a more refined balancing; otherwise, the crankshaft will shake itself loose.
Replace the piston rod. Again, this high stress piece is built for very low speeds and should be replaced with a billet aluminum part. The lighter weight and higher resistance to stress make it an all around superior part.
Remove the governor. Engines are governed at very slow speeds; removing the governor allows you to take full advantage of the engine's power. Remove the oil pan, locate the copper ring that the throttle passes through and use a punch or flat head screwdriver to remove it. Plug the hole with a bolt and O-ring.