Speedometer question... W123 transmission in a W126

[Manbearpigfarm]

So I recently bought a 1984 300SD. The speedometer does not work, and the previous owner told me that he had replaced the transmission with the W123 version which has the mechanical speedometer output, not an electronic speedometer sender. I was thinking about a universal sender to hook up to the mechanical output, but I can't seem to find anything that is advertised to work with a Mercedes. (I see plenty of options for Ford, Chrysler, GM stuff) I've also thought about getting a GPS sender, but I'm not sure that the W126 speedo uses the same type of signal. Does anyone have any ideas for getting my speedometer working without changing the transmission?