Welcome!
As another 1991 K100RS owner I can assure you that you will find pretty much everything you need here, though a word to the wise if yours is a 16V bike quite a lot of what is here only sorta applies, the 16V was a pretty major overhaul compared to the 2v K bikes.
If you do not for any reason find what you need (I also find the search function here something of a challenge) just ask, I have been through my bike pretty much top to bottom and have had huge amounts of help and support from many great people on this great site.