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I'd suggest you start with the link at the bottom of this post. It's related to power lawn mower racing, and may have some of what you need. Another possibility is the go-kart application for Briggs engines. Here's a link that has some technical data, but is probably geared mostly to the 5 hp engines that are so widely used in karting. http://www.compgoparts.com/ Some of what you want can be done through experimentation. As long as you don't intend to mow with it again, you can do several things to make it go faster. I used to have a little tractor that I used to pull my kids on a sled in the snow, and it was a lot of fun. Most of my experience in hopping up Briggs engines was with the 3hp size in the school power mechanics class I used to teach. Sometimes a kid would decide to do a project of that sort, and it was usually with a smaller engine than yours. Many of the techniques will work the same on your engine. First, defeating the upper rpm limit of the governor will gain speed, and possibly a little hp. Governed speed is probably 3600 rpm. It will still breathe pretty well at 4500 rpm. It WILL run faster, but it may not last long once you go past 5000. If you can mill the head to increase compression ratio, that will gain power. I suggest keeping at least .025" clearance between the head and the top of the valves when they open. While you've got the head off, check the valves to see if they are in good condition, or need to be resurfaced. It's sometimes possible to swap or modify some parts in the crankcase on SOME Briggs engines and gain a little bit. You will need more than imagination, a hammer and a file to do this, but if you have access to a milling machine and lathe, you can play games in there. For example, the piston CAN be made lighter by carefully drilling some holes in the skirt. Radius all hole edges to remove burrs, and don't get carried away. This will let your engine rev more quickly. On the little 3 hp engines, we used to remove the camshaft and cut the gear off the cam. Then we'd use a lathe to drill a 1/2" hole through the gear. A cam from a 5hp engine could have the gear cut off, and the end of the camshaft machined to go through the gear from the 3hp engine. The cam has to be machined to the exact LENGTH of the original 3hp cam and the shaft area between the lobes ground a little bit to clear the connecting rod, but it CAN be fitted into the 3hp block. The gear is locked onto the shaft with a setscrew in a hole drilled between two teeth all the way to the center of the gear. Once you have the gear and cam lobes timed correctly, the screw (with locktite) is tightened. That 5hp cam with its greater lift really wakes up a 3hp engine. If you find a cam from a larger engine, It might be possible to do the same thing to your 8hp. The old hot-rodding admonition "Speed costs money; how fast do you want to go?" applies to your situation.
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