Disclaimer: the following is just what I have learned from reading online and my own research. I have no applicable experience at performing a front end swap on either of my bikes. Also there is much more to performing a swap than fitting the yokes & forks - it is just the first step. Swapping a complete front end including wheels is probably the simplest path whereas just swapping triples and forks will involve lots of custom work and plenty of research in finding and fitting wheels, calipers, axles, spacers, etc.
To find suitable fork swaps go to
www.allballsracing.com/index.php/forkconversion
and enter your bike. This list will provide you with bearings needed to adapt any of the listed forks to your bike. But this list also has another use.
Find your bike in the list and write down the bearing sizes. For example a 78 KZ650 is 30x48x12 (IDxODxWidth) for the top and 30x52x16 for the bottom. A KZ440 uses the same.
Now look for good donors. The easiest will be bikes that use the exact same bearing sets. Most Kawasakis from the 70s, from the KE bikes, H1, H2, KZ 305s, KZ400s all the way to the KZ650s, 750s, 900s and 1000s all used the same bearings. So the easiest swap would be to find a complete front end from another Kawasaki from the same era.
Another bike that uses those same bearing sizes are Ninja 250s and 500s (EX250 and EX500) - so swapping a modern front end on to your vintage KZ400 isn't all that difficult.
Another option is to consider are what bikes have fork sizes that are similar to the Kaws.
Most Suzukis (GSX600s - 750s, GSF Bandits, the older GSs, Katanas, GZ250, LS650 Savage, RGV250, SV650, TU250) use the same lower and nearly the same upper - ID is 25mm instead of 30. So swapping any of these is do-able too. The bigger Suzukis (VSTROM, Hyabusa, SV1000, TL1000) all use the same size bearings as the Kaws.
Most Yamahas (R1, R6, FZs, FZRs, old RDs, SR400/500, XS bikes - yes even the XS400, Viragos, RoadStars, etc) either use the same bearing sizes as the Kaws or have the smaller top bearing ID like the Suzukis. Very do-able swaps as well.
There are others like Triumph (new), Victory, Moto Guzzi, a lot of Hondas, etc. that match up well.
Now that you know what will fit you can keep an eye out for a donor front end.
One other thing to consider is the length of the stem and where in the stem these bearings fit on it. Even though a front end may use the same bearings as your KZ400 things may not line up properly. From what I have read the Kaw to Kaw swaps are pretty simple - stem lengths and diameters are the same. As for stem lengths I've read that the Ninja 250/500 swap to a KZ400 (and probably any KZ) is a direct swap. I've seen and read that the stem of GSXR600-750 is a little longer than the KZs but just needs a small spacer.
