Are you really talking about the sensor or the magnet. Mostly the sensor goes on the frame and the magnet goes on the crank. In either case I tape over it with electrical tape around the crank. My crank is gray so I use gray electrical tape. And if it is the magnet you lost, you can use any rare earth magnet, I use the ones from Harbor Freight 10 for $2.99. http://www.harborfreight.com/10-piec...ets-67488.html You can also stick one of them against the existing one to double up if the distance to the sensor is a little too great to work reliably.

Jim