Lawnmower racing is a popular, funny and very real hobby for many people. Once upon a time, it was the result of creative people starting to tinker with lawn equipment, fabricate parts themselves and simply get together for a good laugh. Now it has seen the rise of parts manufacturers, racing leagues with rules and organised contests as well as corporate sponsorship and an almost professional level class of driver. For those looking to get into mower racing, the first step is obvious. You've got to make the mower go faster than stock.
Remove the engine from the body and frame of the mower. This will make all the bits and pieces easier to get at.
Replace the flywheel. The stock flywheel is as heavy as most cars flywheels and is cast aluminium. Replace it with a lighter, milled aluminium piece. This will not only increase throttle response but also increase safety, as cast iron is prone to shatter at high speeds.
Remove the governor completely. This can be found under the oil pan and will appear as a brass sleeve through which the throttle appears. Remove the brass sleeve. Plugging the left over hole can be accomplished with a nut and washer.
Replace the piston and rod. Your lawnmower will see excessive speeds it wasn't built to take. Replacing the stock rod and piston with billet aluminium pieces will make it stronger and lighter. Lighter also means faster.
Replace the exhaust. Believe it or not, aftermarket exhausts exist for lawnmowers. You could also fashion one yourself for less money but that would be near impossible without metal bending equipment.
Put on an after market air filter. Many companies sell adaptors to fit car air filters onto the carburettors found on lawnmowers. More air coming in means a faster lawnmower.
A racing mower requires much more than a powerful engine. Don't think that the stock frame, wheels and tires will hold up to the abuse of a higher powered engine. Reinforce these and buy aftermarket parts as needed. Always wear a helmet and other safety gear when riding.