Pick up a thumb shifter speed selector from a bike shop and mount it to the shifter. Run the cable through the fire wall and attach it to the throttle on the carb, as you would for cruise control. I haven't tried this yet, but in theory it should work similar to the the bike brake being used as a throttle control, but allow you to set/hold the throttle at 1 of 6 settings, if you use a 6speed shifter.
