I had to find a #420 size rear sprocket that I can adapt to the Kawasaki rear wheel. The sprocket from a Honda CRF150R has the correct ID, but mounts with four bolts, rather than the EX500’s six. I think I can re-drill some of the bolt holes and make it work.
I have a 56 tooth rear sprocket, which is the largest I could find off the shelf. By my calculations, that theoretically works out to just over 50 MPH, which I think should be do-able and appropriate.