The cheapest way is to go to a bicycle shop and get a cyclometer. Decent ones are around $30, but you might need to improvise mounts for the sensor, magnet and extend the wires. There is one kit (can't remember the name) where someone has taken a good bicycle computer and made a kit ready to go for motorcycles - cost around $70 I think.
If you plan on racing enduros, you could get a decent enduro computer - can go $300+
A compromise is the ICO Autocal - designed for motorcylces, can be used for enduros but far less advanced than the new models, it has an odometer and speedometer, costs around $170 or so new.
www.icoracing.com
Sorry, can't help you with the tach.