You could rig up a digital speedometer for a bicycle. They're programmable for tire diameter the same as a trailtech computer, and use the same basic magnet and sensor concept, it would just take a little adapting to get it to work. You should be able to mount the sensor on the metal rock guard inside the front wheel that's mounted on the spindle, and then glue the magnet on the lip of the wheel. You will most likely need to put a little extention on the pickup sensor's wire for it to reach from the wheel to the handlebar also, but that's just a simple 2 wire hookup. Just a soldering iron and some electrical tape would git r done.
That's the best idea I have. It would also give you the basic functions that the trailtech endurance has except for the maintenance reminder and maybe some other petty things, and the reaction time is slower, but for mileage it should be just as accurate as the expensive setups as long as you program in the tire roll-out correctly. You could probably get one for around $15-20 at walmart or sporting goods stores. I think most of them are, but make sure it's waterproof or it would probably be short-lived on a quad.