Chopper Gear

you need a gearbox
you can get one that attaches to the place where is the front sprocket and a large rod that covers the original tree that you can set a larger sprocket on the front and the box gives you a lower speed and higher. I have one fitted to my Mini Crosser gear, but i cannot change while moving, but you might be able to get one with a clutch somewhere. Sorry I do not know who sells them everywhere.