At a bicycle shop you can get a multi-function speedo/odo for around $30, but you will probably have to extend the wires and custom mount the sensor. The Cateye brand is supposed to be very good. On the JustKDX page, a guy recently posted instructions on how to run the wires up through your stock odo cable.
ICO sells the Auto Cal speedo/odo which can also be programmed for enduros for $160 (more if you want the thumb operated switch).