Nice K5. If you get a grab rail and want to be as original, get one specifically for a k5 or k6. The larger, newer style turn sigs for a k5-6 have a little different style mountings that match the k5-6 rail.
The mounting stems for the k3-4 will work best with the k3-4 rail.
Only diff between the two rails is hole size for the sig mounts, and that dictates which style sigs you can use and their matching hardware. With the correct headlite ears tho, you can use either style front sigs/stems, k3/4 skinny style, OR the k5/6 fatter style.
Went thru same scenerio with my k5, as previous owner had installed a padded sissy bar that also employed the rear sigs.
The bumper was long gone. There seem to be a lot of them on ebay.
NOS are just about non existant. Love the flake apricot red. Used to have one just like it.