First off what are you wanting to do? Are you wanting to lower engine RPM at Highway speeds or performance stoplight to stoplight? To Lower our RPM you'll need a sprocket with fewer teeth at the rear, more at the engine or both. Don't know whats available, but by checking into your bike nomemclature you should be able to to know where you start. Doing just the front will probably involve less cost and you may be able to use your existing chain if still serviceable. If you figure the ratio of front sprocket to rear you can use this when you determine how much you want to lower your cruise RPM to buy the the correct sprocket or sprockets to get this reduction.
However now I offer a disclaimer. If you lower your cruise RPM your pick-up will suffer some. It's kind of a balancing act holeshot versus cruise. I say lower your cruise RPM, get a cam, a pipe, open up the intake and re-jet to get back that lost power.