I have a '91 250, so its probably almost identical to your bike.
I used a UFO part for 1990-1991 KX125/250. From Hi-persports.com it is part 116275, and is $16.99. The finished product is hard to tell its not stock. A picture of the fender on my bike is here: http://members.home.net/caidin/kdxpix.html
To install, you remove the old fender and the two steel fender support brackets (I did not re-use these). The new fender only required a little cutting on the left and right side edges with a knife or coping saw to clear the subframe rails where there are some metal tabs causing interference (not visible). You also have to drill two holes in the fender to allow mounting of the two large screws (these are the allen head screws that are about 1" from the seat mounting screws, but are on the inside of the fender in the wheel well). The hole will be bigger than the screw to fit over a round flange on the subframe. Also, a small trim is needed on the rear corner of the sidepanels for clearance.
Once the fender is mounted, you need to support the rear of the silencer. I did not use the existing silencer rear mounting hole which is seen in the photo. Instead, I made a metal strip going from the large allen bolt mentioned above, down to the flat side of the silencer closest to the rear wheel. I then ran 4 screws through the bracket into the silencer body. The bracket was 2" wide, 5" long, 0.1" thick aluminum. I used RTV on the 4 screws (actually I used 4 little bolts) so exhaust goop wouldn't leak from the screw holes. Be sure the tire clears the whole silencer and bracket when suspension is bottomed. I have run this setup 1 year and it works great.