Well this could work if what SS109 is referring to is the 2003+ KX stems being longer. As long as the 1992-2002 KX stems are close enough to the KDX bearing locations, the upper bearing spacer sleeve idea might actually work! I'm curious particularly for a 1996-1997 KX125 or 250 triple clamp as it will work on my 1997-2004 KX500 forks. Although I have the KX500 triples which have basically a KDX steel steering stem, so they are a direct bolt-on. Some people have pointed out that the KX125/250 have a different offset than the 500, and 500 owners preffered the 125/250 offset better. KX owners is the key here. The 97+ KX500 was basically just a novelty BIG big bore play bike at that point, as there were no longer open classes for these suicidally fast machines to run in at sanctioned races.
Also according to KX Guru Racing, 1997+ KX500 stems are a larger press fit diameter than 1996 & earlier...
KX Guru Racing also sell billet steering stems for the older 500's whose steering stems don't as easily swap into the newer choices of upgrade forks without the knurling etc.
Emig Racing also makes custom steering stems per customer spec.
Personally the knurling to make a slightly sloppy fit into a tight fit somewhat bothers me, but with permanent cylindrical bonding spec loctite or other brand retaining compound, I doubt anyone would have trouble with a knurled stem.
SS109, I was checking out the RM125 steering stems, which look just like some Suzuki GSXR steer bike stems I have as well as V-Strom stems I've browsed for street bike fork swaps into vintage Japanese bikes. On the conventional 49mm Showa RM fork triple clamps, I think I can just chuck the triple into a lathe and cut the step off the bottom of the triple as it has a cast in step that rises up a good 8mm or so above the triple before the bearing seating area.
I just checked, and all of the RM 2-stroke Showa fork's triple clamps through 2006 have this raised step on the lower triple, so they all should be able to be machined down slightly to lower the ride height and fit them into the KDX frame's head tube as described below.
I think I can just cut this down to almost nothing, which also lowers the ride height increase of the KDX's new fork swap, and cutting the stem back to the same press fit o.d., and then run the aluminum stem RM125/250 Showa fork's triples direct into the KDX frame.
Although the necessary 30x47x12 upper bearing would be riding 3.5mm into the threaded portion of the stem approximately if ran with a 3mm spacer under it to be at the top of the KDX frame's bearing pocket to keep the top of the bearing flush with the frame's top, or else run it down in the frame and use a 30.01mm i.d. spacer above the bearing 3mm thick to take up the slack, probably the more appropriate solution considering the threads location and 12mm tall upper bearing vs 15mm original. I'll have to look and see if the later Showa 47mm inverted forks have a similar raised step portion on the lower triple bearing seating area that could be shaved off to stretch out the stem to near KDX dimensions.
*EDIT - regarding RM125 Showa Twin Chamber 49mm & 47mm forks 1996-2008, The machining I described still does not quite make it, but through looking back over some of my Suzuki vintage street bike fork swap info, I revisited the V-Strom DL1000 triple clamps which would work for some of my vintage street bike swaps. They have the same type of aluminum steering stem with 30 mm ID bearings as what the RM Showa forks do, as well as GSXR & Hayabusa models 1990's & 2000's...
The VStrom DL1000 steering stem is just a bit longer and will swap perfectly, I just got One in the mail today for $2.98 plus shipping as a steering stem donor. Coincidentally the shorter RM or GSXR steering stems that I have, swapped into this V-Strom triple clamp, will also solve my street bike fork swap problems! The RM stem has a much taller press fit area, but when swapping onto a KDX, you machine that step off of the bottom lower seat area anyways, so you would have to cut down the RM stem regardless. One member bored out the triple so that the stem sat up higher, as well as milling off that raised step. I don't like the idea of less and less press fit area, I'm okay with milling the top flat, but recessing it from the bottom as well is too much. These v-strom stems are commonplace for cheap. Non-ABS models, it has to have the aluminum steering stem, ABS models have a steel steering stem for some reason. Newer and different forks.
RM250 used Showa suspension 1996-2000, but lapsed a few years & ran KYB instead. I'm not sure if the triple clamps were the same, But it seems to be a Suzuki steering stem theme to use the aluminum 30mm upper and lower bearing stem from the mid 1990's & on.