Not sure if 1990 or 1991 clamps will fit but if they do (stops?? stem size may be off??) the 125/250/500 all are stamped 125 250 500 etc on the underside but are all the same parts, I only found that out as one of my bikes had KX 500 clamps on it so asked questions on here!
That has worked out nicely for me as have ordered some of the KX GURU clamps from Oscar that have the same stops and off set etc to fit up some late forks and wheels on the bike! If you want me to take any measurements of the clamps shout as got them in hand, if the 90 (one year only) /91 onwards clamps will work for you the 1990 forks are gold (UK any way) and are 41mm, the 91/92/93 possibly 94 are 43mm and blue, if you want it to look era correct have them re'anodised silver at the same time you rebuild them, another thing use SKF seals don't skimp there, other years may also work, I think the 95 onwards are silver but I cant help as to what years stops and stems etc might work so might be worth you asking others on here!
Obviously if you want to go all the way with the GURU clamps and later KXF forks you will also need to think about the front wheel too as later models have thicker axels and different spacing 👍🏼
http://www.kxriders.com/forums/index.php/topic,17396.msg137840.html#new 👌🏻