Sprockets are the cheapest performance gain that you can make. Start with a 13 or 14 front and a few different rears like 40,41,42 with a 20in. rear tire and try them all. I have used many different combos over the years. You may have to take into consideration chain adjustability depending on the swingarm you are using. Your engine build will determine what you need. I had been using 14, 40 on the track but when I had to use my stock head and lost 1/4 of my compression the 13,41 was the better choice. A 15 would be used with a high performance build or 18in. tires.