As an alternative to swapping, check the DIY section for repair information, should the issue be the odometer not working. FYI, I did a write up on this some time ago, and others have covered this repair on the older, mechanical speedomter versions.
http://www.peachparts.com/shopforum/showthread.php?t=206598&highlight=odometer
You might also be able to use gears from the donor to fix the broken one, but more than likely they are on their way out anyway from years of use.. Of course, the cluster has to come out in any event, so you could change the entire cluster, or just the speedometer and/or tachometer too. These clusters, and the instruments themselves, are extremely simple to take apart and re-assemble.