If it wasn't for bad luck, I wouldn't have any luck. Found some rings on ebay, outbid at the last second (actually 3) by $2.50.
I started looking for some oversized pistons and rings and thought I had them, from a dealer in Indiana. Got a call from them today and they were wrong, they didn't have them.
The guy told me that Kawasaki has a strict 10 year rule. Nothing is made for bikes after 10 years. Scares me a bit when the manufacturer thinks their bikes are crap after 10 years - they could be right.
There are a bunch of big bore kits out there but the price is much to high for this bike since it needs so much to make it right.
Actually, it is federal law that mandates vehicle manufacturers must make parts to keep them going for at least ten years. That doesn't mean they stop making parts then, they just aren't required to. I just saw a 750 twin valve cover gasket at the dealer which could not have been more than a few years old, and those bikes have been out of production for about 20+ years. You'd be surpised what parts are hanging around, and the good news is: metal parts don't age and go bad like rubber parts and gaskets do.
Before you get too depressed... my 750 twin is 29 years old, and they made a lot fewer of the twins than the four bangers. Even so, I just found another new set of +0.50 pistons and rings on e-bay for it.
The KZ650 was pretty popular and I would not be surprised if there are NEW piston/ring sets around. But, given your cost limits, a bore might be too expensive. My shop charges about $75/hole, so about $300 to punch four sleeves. You might not need that.
I don't know the condition of the pistons you have, they might be servicable if you clean them up. They are aluminum so vertical ring groove wear is their wear point. If the bike is relatively low mileage, they might be OK to use. use some chrome polish and a toothbrush and give them a rub.
The cylinder walls are another issue. If they are rusted, and if it's only surface rust. It's possible you could hone the cylinders and install a new ring set and run the engine a whole bunch of miles.(?) I couldn't tell you how many cars have gotten the "ring rebuild" to get another 30k out of them before they get junked.
If you want to check the cylinder walls, get some #400 and #600 wet/dry sandpaper and light oil and sand them a bit and see if they clean up. If so, a hone might get them good enough to run again. JC Whitney had a nice little hone for about $20.