I’m sort of kicking around the idea of sourcing K1100RS fairing parts for my bike as I really like the look. At this point, I’m just gathering information and thinking about it. Going through the parts fiche, I see that it actually came as a kit/upgrade way back when.

The upper fairing and rad trim panel are common between the 2 models, so it would appear to be straight forward as all the mounting points would line up. But I have a couple of questions for those with far more knowledge than I.

1. The left and right knee pads are different parts between the 2 models, but it appears as though the K100RS knee pads will fit the K1100RS left and righty lower fairings. Correct?

2. The belly pan attachment is different, so I would also need the bracket that attaches the rear underside of the belly pan to the engine?

As long as the left and right lower fairings are complete, I need 4 pieces-belly pan, belly pan mounting bracket, and left and right lower side panels.

I did the conversion on the K100RS in my avatar photo.  You are correct about the upper fairing and radiator grill.  The rest are all different. 

You can do the 1100 fairing without the battery covers.  It will save you about $150 on the job.  The knee pads are different for the 1100 fairing, the 100 knee pads have a different mounting and won't work with the new fairing.

The bracket for the belly pan is frequently missing when people sell the belly pan.  They don't think to remove it.  A replacement is close to $70.  I made my own out of a piece of 0.100" stainless.  All it is is a piece about 30mm wide and about 25cm long with a couple bends and four holes two of which are threaded.

If you are interested, I have most of the parts to do a conversion sitting in my basement collecting dust.  I'd(actually my wife) like to get them out, so I would let you have what you need for the cost of shipping and maybe a couple bucks for my time packing them up.  They are all in good shape and painted a shade of grey sort of like graphite, i don't know the actual color, I didn't get the cowl with the paint tag.

Easy peasy. I went for the K11RS side panels when I made a K75/1100RS.

This is using a K75 engine and K75 frame with a K100 tank.