CDave - I'm sure I'll get flamed for this comment, but bicycle computers are best left on the bicycle.
If you need an odometer for riding enduros, it's hard to beat the stock KDX unit. Yes, it's not very accurate, but it can be easily reset to any mileage. Bicycle odometers can only be reset to zero, although I've heard the Topeak unit can be advanced, it sounded like an awful lot of work. If you're a serious enduro competitor, then take a look at what is being offered by ICO, Pacemaker or Watchdog.
If your stock odometer no longer works or you feel you absolutely need to have something that tells you how fast you're going, then buy one of those cheap bicycle odometers from Radio Shack, K-Mart, Target, etc. However, if you ride a '95-'01 KDX, ditch the sensor and magnet and buy Fredette's heavy-duty sensor and rare earth magnet. Properly mounted, these items can be used without a disc guard, will outlast the cheap computer and can be used with REAL enduro computers.