1998 KX 250 Forks

I'm trying to do the opposite: 99 forks on a 2003 KX.  By saying "front end", I assume you mean complete forks and triple clamps from 2003 to your 1998. A friend just picked it up from me to assemble and we were comparing clamps.  The fork diameter is different between the 2 so a simple fork swap would need modification to use the same clamps.  The bearings are the same (verified on Partzilla) but it looked like the 2003 stem was a bit longer so you may have to shim it to make it fit tightly (not advised for MX).  It could also be that the height difference was due to the clamp thickness being different between the 2 years.  Unfortunately, the bike is already back together so I can't measure.

So, the final answer is it can be made to work by putting the complete fork/clamp assembly from the 2003 to the 1998 but there may be some modifications needed to get it spaced properly.  Your decision if the risk is worth it if you want to perform the switch.  For me, I'm old and slow now so I will happily assume the risk!

Eric

Better off rebuilding, revalving the older forks and upgrading front brake Caliber for the cost and p.i.t.a that swap will be 

I remember my 98 forks being really good for the time. I also don’t recall anything earth-shattering being upgraded through the end of the two-stroke line. Unless I was swapping for something like KYBs off the 09-12 KX450, I would just fix up the 98 ones (assuming they’re in serviceable condition).

I never had an 03' but I liked the suspension on my 97' and 98 better than that of the 04,05 and 07 I had.  My 97 Suspension revalve from ProAction is one the best suspension packages and balanced bikes I ever had.   I just can't see the swap as an upgrade.  

You couldn't ask for a better Off road fork that the stock '98's. Super Cush.

I pretty much agree with you as my old 99 forks (similar to 98) were replaced with a set of forks that were revalved/sprung Factory Connection that I bought off of "ee-baye".  Even though they were not set up specifically for me, they worked very well for mountain singletrack here in CO. The only problem was when I pushed the pace hard in some fast/tight/downhill trails, they flexed a bit under my weight (230 lbs at the time) causing a bit of vagueness.  I rode them back to back with a bike with FC tuned 48mm AOS forks and they were more precise with action that was just as good as my 46mm forks.  Full disclosure: At the time, I was mid 40s with a background of "A" class in east coast hare scrambles/enduro.  That is the basis for my assessment.

If the writer of the original post is heavy or his 98 forks are shot, he may be better with the 03 forks if they are in good enough condition.  If the 98 forks are in good shape, there is no advantage to the 03 fork swap.

I just found someone who measured the stem difference between the 98 and 03 and it is 6mm.  Just thought I would provide and update.