I have seen some exhaust set ups on the DIY go kart forum that are close to stock sounding, but still open the exhaust up for performance. The stock 'spark arrestor' basically blocks the flow to stop any sparks from exiting the engine, this creates a huge flow block that tends to get packed with carbon and soot. I will see if I can find a pic of one.
Here is a decent kit that will set up you up nicely. Of course you can crazy and do rod, flywheel, cam and so forth, but this would be the best improvement for the money, and the stock style briggs screw in muffler won't be obnoxious.
http://www.ombwarehouse.com/Predator...-and-More.html