Look here first;
https://www.ninjette.org/forums/showthread.php?t=39749
You have but a few options;
Koso, Trail Tech or other brand (dash replacement) About 100-125 + install time
A bicycle speedo can be wired on (prices are all over the place)
Use a GPS speedo (smart phone or gps device)
A taller front tire can also be used (least favorite option, included for completeness lol)