For better acceleration, either add teeth at the back sprocket, or lose teeth at the front sprocket. The difference in size of your sprockets will determine the difference in effect. Generally losing 1 tooth of the front is equal to adding 2 or 3 at the rear.
the 500R isnt built to go fast, its built pretty much to learn on, and corner well, not much else. though if you want to make it look and sound much better, put a Muzzy pipe on it, I know I LOVED mine.