Bill, sorry Ron Ayers couldn't help you, I checked a couple other common Concours sources we have here, no luck...
they still show as active on Kawasaki's parts lists for both the Concours and the Voyager models.
The last time I bought those was around 2006, and they were available then.
I think I may have a couple in my parts bins; I will get back to you if and when I do, even if I had an old one (which I always save when I replace stuff) I could measure it, and convert it to a common O-ring number.
the other alternative, which I highly recommend over dealing with the air assist forks, is to replace the springs themselves with Progressive Springs form our merchant Murph
Murph's kits
http://www.murphskits.com/catalog/product_info.php?cPath=1_97&products_id=52they are affordable, and when installed, there is no further need for the air assistance. Then, you could basically place a strip of rubber cut from an innertube around the area on the fork where the crossover existed, and install a hose clamp, this will prevent oil loss.
I personally installed those springs in 2 of my pre 94 model bikes, and they improve the ride dreastically, without having to spend a ton of money.
make sure you check for messeges, and keep that feature enabled here, I'll get back on the orings a.s.a.p.