All K bikes model year 1986 and later use the same fuel level sender. The RS and RT/LT bikes have space for a 2" round fuel gauge and if you're careful you can add one to the side panel of a K75S. The "naked" K75/K100 models and the K75C have no place to mount a 2" dial gauge unless you mount it on the handlebar.

After doing some research I came across two aftermarket LCD fuel gauges that will work on K bikes.  They're really the same fuel gauge but marketed under two names:

1. KOSO 

 

© 2016 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.