The only thing I can really think of is a new silencer (exhaust, whatever) and maybe a set of reeds. This will add some horsepower, change air flow, and increase throttle response. You'll notice the difference, and not to mention it'll sound cooler too. You could maybe do some clutch stuff to it too if you wanted, but that's up to you. If it sounds to hard to you maybe you wouldn't want to try.