1984 KX250 has single downtube frame due to angled exhaust port of motor. KX500 1986-2004 all have centered exhaust port and will not work with modifying the frame.
Dont think 1985 KX500 will work either but cant remember. 1984 KX500 and earlier you are on your own.
1983/1984 KX250 and 1984/85 KXT250 Tecate are what fits.