1. Replacement wire harnesses, complete with a new sensor, are available for many computers. Considering the low cost for the most basic wired bike computers, this option might not be cost effective. Hostel Shoppe has extra long wire harnesses, that you might find more appropriate for recumbent use, listed for several common computer brands.

2. I've extended the sensor wire for several computers by soldering in a section of speaker wire. You'll still need a sensor to replace the one that you said was stolen.