www.egauges.com does sell a Hall effect sensor for the VDO electronic speedometer, then all you need is a bracket to hold the sensor and an exciter on a CV Joint. There was a company that would modify the CV joint flange (basically machined a bunch of 1/4" teeth into it) that became the exciter. I bought one for my VW about 10 years ago that has worked great. Can't remember the name of the outfit though. They would have about a 3 page spread in HotVWs all on an orange background. Probably out of business now.
EDIT: Sure enough, Barrett Enterprises was the place and they are out of business AFAIK (http://members.aol.com/barrettvw/ was their web address but AOL Hometown is gone too.)
Might also be able to make a toothed ring out of 16 gauge sheet metal that could bolt on to the CV.
I'll post some pictures up later and see if I can find the instructions that came with it.