There is a tech tip here about this on the site somewhere... Anyway, I just went to my local Schwinn bike shop, they had a range of speedos from cheap to expensive, depending on how many features you want. There are other brands as well.
All they are is a display unit with a wire to a sensor, and a magnet. You mount the magnet on a wheel or rotor ( not a problem here ), and position the sensor so it passes over the magnet. On my Recon, I put the magnet on the front wheel, and made up a small bracket mounted to a bolt on the back of the brake drum. I had about a 3 inch long "c" shaped piece of aluminum leftover from a truck running board, and I just cut it until it would put the sensor where I wanted it. I chose the rear area of the drum so as to minimize damage from sticks or rocks. The magnet is epoxied to the wheel. I did not wish to alter quad or plastic, so I simply ran the wire up the frame and mounted the display on the front rack's rear bar. If I choose to remove it, no one will ever know it was there. Most speedos have simple calibration instructions. Once mine was working, I checked the odometer versus my riding buddy's SP 500, and slightly tweaked the calibration reading to suit. I have put about 600 miles on it over very rough terrain and thru 3 feet of water, and it has worked great.
Sorry to ramble on, the tech tip probably explains it better.

 

Recon98,
Your info is very helpful and I thank you.I will be looking into this speedo mod this wk,
Bill