I'm part way through fitting a K1100RS front end (for better suspension, brakes and ability to fit a 17" radial tyre) to my 1983 K100RS. After fitting the new front end I hit a problem when trying to fit my 1983 K100RS handle bars. The cast handlebar mounting base clamps on the K1100RS front end are spaced slightly wider and do not allow for the K100RS bars to be fitted firmly and snug. I had to file a small amount off the outside side of each of the cast Ali bar mounting caps to allow for the caps to be properly aligned and clamp down evenly and securely. Not difficult, but be aware that you will need to do this mod if fitting K100RS bars to a K1100RS front end.

I just use K1100RS handlebars.

K75S/K100RS bars are a tad too narrow in the middle as you mention:




K1100RS vs. K75S/K100RS bars:



Another issue is that the 4V upper fork tree pushes the dash pad back which makes the switches harder to use if you are using an OEM tank bag.

Custom bracket to move the dash pad forward.



On this bike I use the square dash pad because it has four switch slots and a power outlet for my electrified tank bag.