Have you tried posting at mtbr? I figure someone there would know of something that would work.
I've seen them set up some sort of timing system at the PAL track using a laser, I am guessing, at the bottom of the starting hill to see who could do the fastest gate start.
typed in timing system on ebay and saw this
ACCUTRACK Track & Field Timing System ~ Speed Trap ???? - eBay (item 300291279735 end time Feb-08-09 08:47:36 PST)
how about this???
Tutorial: How to Make a Cheap Personal Lap Timer - Mini-ZRacer.com Forums