Some years ago, I raced Go karts! Here are the most productive and cost effective ways we added to our go karts! Granted these were fully built $750-1000 motors, and even then, they only produced around 7.5 hp!
- air breathers! The constrictive air breathers of a stock briggs 5 hp can be opened up using a "K&N type filter"
- exhaust. The most effective was a straight pipe exhaust with a bend in it to still hold back pressure! Made the motor really load though! My suggestion would be a pipe with muffler, still load, just not as bad!
- Amount of oil used! Most briggs require much less oil than the manufacture suggests! Although running the chances of blowing, we would reduce the amount of oil down by several ounces and also the weight! It requires much more frequent oil changes, but also produces more power via less drag!
- Changing the carb! A little more expensive mod, but can be the most helpful! Bigger carb means more fuel, which equates to more power in the long run, especially if you do the other mods!
Hope this helps, and If you have any questions feel free to ask away!
One place you could get the air filter, exhaust, and carb would be here:
https://www.cometkartsales.com/store/air/fabric.htm air filter
https://www.cometkartsales.com/store/p&h/briggs.htm pipe
https://www.cometkartsales.com/store/p&h/rlvbrigg.htm muffler
https://www.cometkartsales.com/store/briggs/carbdiap.htm carb
https://www.cometkartsales.com/store/briggs/images/briggsintake.jpg filter adapter for carb