There are several after market speedo cable suppliers that offer cables in long length that you cut down to fit your applications. I went with one on my Ducati and it was simple the square end had more than 6 inches I could trim off. I think I cut off 4 inches to make mine.
But if it were me, I go with something like the Vapor Electronic Speedo tach combo from Trail Tech. I pot one on my KLR a month ago and love it. Plus it's a smaller package that the stock gauges.
Much easier to install and a lot more bells and whilstles on it. And they are under $130. Plus they are small and easy to install almost anywhere.
You get all digital Electric Tach, speedo, clock, engine temp, trip, odo, stop watch, programmable shift points and ambient temperature. I really like the programmable part. I have fine tuned mine to give me economy shift point. (Which I love to ignore at times!
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AND you can programe it to warn you of engine overheating.
Vapor Digital Gauge at Trail Tech Home