If your bike, swing arm, axle, and spacers are from an 86 then you know what you need, 86-88 KX 250 or 500.
To verify, measure the distance from the inside to the inside of the swing arm where the axle goes through.
Minus your combined spacer widths.
Your figure should be close to 145MM.
It can be a little bigger or smaller.
Hope this helps.