the early inverted kx forks were 41mm until 91 when they went to the 43mm. They stayed 43mm through the 95 model year and went 46mm in 96.
so you have :
90 = 41mm
91-95 = 43mm
96-00 = 46mm
01-present = 48mm
You are going to need to find out what exact size they are. A lot of the 43mm forks were annodized blue on the upper tubes.
But if you find later model forks, look at the kawasaki online fische. If your year and the year of the forks you are looking at use the same size bearings, you maybe able to use the entire triple tree - only thing you may need to do is to press out the stock stem into the new lower clamp.
If you do it that way get the entire front end with the brake caliper, wheel and axle. These changed through the years!