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
Or you could always mount a GPS receiver to your bike. Shows your speed as well as helps keep you from getting lost! As far as the tach, go to www.tinytach.com There isn't much on their site, but it gives an e-mail address for more info. I've also seen their ad in the back of one of the MX mags.
Forget electronic speedo kits that are packaged for bicycles, their mounting parts are not big enough for the motorcycle spoke. A GPS doesn't always work perfectly, like when you are next to a big solid object, like a cliff face, it'll block some of the satellites. The big speedo kit that someone mentioned, that is packaged for a motorcycle can be purchased at http://www.rockymountainmc.com/home.asp?sid=00261434