easiest performance boost is to file about .005- .010 inches off of the exhaust valve stem (check it during a full rotation of the cam to make sure you don't take too much off). This will allow you to get the full mechanical compression rather than about 70% of full compression due to exhaust cam lobe profiling done for ease of starting. Also, you can do a few tricks with filing the key on the flywheel to advance the flywheel timing and creative mods to the points to advance the points timing 3 - 5 degrees to allow a little more pop.
My company sponsored a racing lawn mower for a few years and the first one we built up for one of our employees to race - it was a lot of fun and you would be amazed at what a mower looks like running 30 mph into a corner spraying rocks and dirt everywhere as they pass on the inside trying to get the hole shot
