If all you want is extended fork tubes, get in touch with Forks by Frank, ie
http://www.frankmain.qpg.com/
It ain't cheap, being about $200 for a set of tubes, but they are beautiful pieces, machined to fit your specific model, and done in polished stainless. I don't have a set on mine, but I know guys what do. They are awesome tubes and beat hell outta anyone elses for fit, finish, and overall value, not to mention that he can handle bikes that no one else touches.
The standing advice is to hit the website and call the number there. I've heard that he's not too fast responding to e-mail, but phone works fine.
As an aside, the kz900 is a bad-ass ride. I love my Hondas but I'd snatch up one of those old KZ's in a heartbeat if I could. Hit this site:
http://www.kawachopper.2ya.com/
I find the paint a bit busy for my taste, but the execution is awesome and he documents the build in photos at pretty much every step.
Last option to mention is to simply cut out the headstock and replace it with a freshly machined stock in a configuration you are more familiar with (Harley or Honda being the two I find easiest to find forks for). Lowrider Tommy can get machined necks pretty cheap. Actually, he can get all kinds of cool gear for import bikes and happens to be a major kick-ass builder in his own right. Tommy is a stand-up guy and one of the coolest people in the business. His site is
http://www.lowridersbysummers.com/
Good luck on your build! If you get the chance, post some pics of your progress.