Kawasaki parts numbers and interchange are as follows:
SWING ARM
FITS 90 KX500 - 33001-1347-CE
FITS 91-93 KX500 - 33001-1371-CE
REAR BRAKE CALIPER MOUNT
FITS 90-93 KX125 KX250, 90-91 KX500 - 43044-1092
FITS 92-95 KX500, 94 KX125 KX250 - 43044-1111
REAR BRAKE CALIPER
FITS 89-90 KX125 KX250 KX500 - 43041-1286
FITS 92-93 KX125 KX250 KX500 - 43041-1482
I am running a 02 kx500 swing arm (96-02 KX500 - 33001-1496-TX) on my 88 kx500 frame, however, I also needed the rear brake caliper (95-96 KX125 KX250, 96-03 KX500, 96 KLX650R, 97-03 KLX300R - 43041-1577) and mount (95-96 KX125 KX250, 96-04 KX500, 96 KLX650R, 97-07 KLX300R - 14079-1105), as well as the shock linkage from 90-04.
So can the 90 swing arm fit your 92 frame? Sure, but you will need the brake mount (90-93 KX125 KX250, 90-91 KX500 - 43044-1092) and caliper (89-90 KX125 KX250 KX500 - 43041-1286).