Here is something I posted on another site. Your 100 seat will be about the same as this one. I would only take 1 1/2" to 2" off of a 100 seat.
Is your CRF450X to tall for you? One way to fix the issue is shaving the seat. You can get as much as 2 1/2" out of the seat. One thing that you do need to know, removing a lot of foam out of the seat can make it not as comfortable for those that sit a lot when riding..
First you will need to remove the seat. This is simple, just use a 10mm socket to remove the two bolt that hold the seat on. These bolts are right by the back of the seat. You can see them in this picture. After removing them slid the seat back and pull up to remove it.
Next thing you will want to do why you have your seat off is make sure the seat bolt that is on the tank is tight. A lot of people have this bolt and washer come off. Even if you're not shaving your seat check it. The bolt and wash screws into the tank.
Now that you have the seat off, remove the staples out of the bottom of it. A small screw driver and some needle nose pliers will do the job.
Once you get all the staple out, remove the cover from the base of the seat. Make sure you don't remove the foam off of the base.
Next you will want to mark out how much foam you want to remove off of the seat. You will want to mark the seat in a number of areas to make sure you cut the foam even. Take your tape measure and measure from the front of the seat to where you want to start shaving the seat. You will need to do this on both sides of the seat. Once you have them marked pick the spot to where you want to end the cut. Measure from the front to this area and mark it on both sides. Then mark on the seat about ever 2" on the area you're going to cut out. This needs to be done on both sides of the seat. These marks will let you measure from the bottom of the seat up to the mark on both sides. This way you will know the cut will be even on both sides. Mark on the sides of the seat the area you're going to cut out. Then all you need is a good turkey carving knife to remove the foam from this area.
Now that you have it all marked out, it's time to start carving out the foam. You should have your lines on both sides of the seat. Just stay right above the lines on both sides and remove that foam. It will make it a lot more easy to cut if you hold up on the area you cut already. The trick is taking your time cutting it.
Now that you have cut out the area you wanted to remove, you will need to clean it up before putting your seat cover back on. This can be done with a file or rasp. Once you moved any lines that the knife made you might want to spray the seat with some scotch-guard. Let the scotch-guard dry then it's time to put the seat cover back in.
The seat cover might be the hardest part of it. You need to make sure you get it on tight. Start in the front and work your way back. Each time adding just a few staples. The reason for just a few at a time is, if you need to re-adjust it, you will need to move the staples. Once you get the seat cover on tight finish stapling the seat. Make sure you put enough staples in it.
Will any staple gun work. I used a few before I could find one to do the job right. I don't know of any hand stapler that will do the job. It doesn't set the staples deep enough. I got one from Harbor Freight that hooks up to a compressor. It work real good and it was cheap. Here's the one I got.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=46365
Now that the seat is shaved and the seat cover is back on, re-install the seat. Make sure you get the front of the seat in the bolt that is in the gas tank. Then put the two seat bolts in and you're set. Now you might be able to touch the ground with your feet if you have short legs.
