Try Kosman Racing in CA, 415/861-4262 for a widened rear rim. It makes it so you can run many more of the popular radials, which I liked on my '87 K100RS Motorsport. They modified my rim about 8 years ago.
Luftmeister also had Kosman widen rims for them and then they resold them so you may be able to find used widened rims out there somewhere.
Another way to go is to use rims from say a '91 4 valve K100RS which is the necessary width in the rear and the front is wider also I think. It has a three spoke pattern which you may like better. Life is choices!