These guys make chain drive kits. If you want to be like a 6 speed I think you would want a 34 tooth sprocket. With a 27 tooth sprocket you would probably be running around 3500 RPMs at 65 mph. This 6 speed at 2770 RPMs is about 70 miles an hour. It varies depending on the model of bike. If you have the owners manual it should tell you what teeth are on the engine sprocket, clutch, transmission and rear wheel. Then you can go to this website. Big Boyz Head Porting - Gear Ratio Comparator and Belt Calculator
Then where it has the option to put in alternate gear ratios put in what you're has and it will tell you the ratio in each gear. It will also tell you the miles per hour for each gear at any given RPM you decide. This will help you figure out what exactly you want.