I used #95 Keihin small head main jets. Jeff at Z1 can get you squared away. I think you could safely go with 97.5, but 95 should be plenty. My stock jets were #92, yours should be the same (atleast according to the parts diagram at kawasaki.com).
Changing the main jets is pretty easy, especially with the air cleaner off. Take the gas tank off, making sure the petcock is in ON or RES (or OFF if you don't have a vaccume petcock) before removing the fuel line. Remove both throttle cables (this takes more work than ANY other part of changing the main jets, imo), I usually unscrew them at the handlebars to give myself a little more room to play. Loosen the clamp screws on the intake boots, and carefully work the entire rack off. There will probably be a little gas left in the bowls, somthing to keep in mind when handling them. I wasn't concerned, gas is far from the worst thing my benches have seen. With the rack upside down on your bench, unscrew the 4 float bowl screws and pop the bowl off. My gaskets weren't that old, so they were still useable, yours may or may not be. The stock jet will be a little hex headed bolt with a tiny hole in it, you need not remove anything else to get to it, its right there pretty obvious. Make sure to hold the needle jet still, one of mine tried to turn instead of the jet. after your old jet is out, set it aside (preferably in a container so you don't lose it.) and screw in the new jet, *snug* it with a flat bladed screwdriver. Repeat for each carb, completely reassembling them as you go. Check the parts diagram at kawasaki.com if you need a visual breakdown.