The 91 KX125/250 triple clamps will swap to the KDX frames, but 92-95 KX125/250 triples will require a stem swap, although those years stem swap may be the exact same press fit diameter as the KDX stem (most '96+ upgrade forks use a steering stem several thousandths of an inch larger, making the KDX stem no longer a press fit, and requiring knurling to increase the o.d. slightly on the stem, + retaining compound.
It's also pretty well stated all over the forum that KLX250/300/650 triples through at least 2007 should all be the same clamping diameters as 91-95 KX125/250/500 forks, and I believe from references on this forum, that 91-94 KDX250 triple clamps share these same dimensions. Most KDX250's (exact same triple clamp as the KLX650R) use a lighter aluminum steering stem also, for a minor weight savings
So the KLX or KDX250 triples can mount the 91-95 KX125/250/500 forks as a direct bolt-on, and 94-95 are mid valve forks, a bit more advanced progressive damping that helps for higher speed riding and racing, and also have a 20mm axle and bearing size, which increased tracking and flex resistance. The bearing and axle may have jumped to 20mm in 93, but there are conflicting sources on the internet.
You'll of course need the KX front wheel as well, as it has a larger diameter 17mm axle or 20mm axle (depending on year) which will give you another advantage over the 43mm KLX forks & the stock KDX/KLX 15mm axle in terms of front end rigidity, as well as the external adjustable rebound and compression of KX forks vs KDX/KLX, + the 94-95 having additional compression stacks on the mid valve, for more progressive high speed damping vs previous years with only base valve compression stacks (perfectly fine for casual woods roding, but mid valves help out in hard fast racing use, desert riding, and motocross).
The KX500 triples these years will be a direct bolt-on to the KDX, but 91-95 / 96 / 97-04 KX500 forks and triples are scarce to come by...
94+ ('93+ perhaps???) KX125/250 use a 20mm front axle, so the wheels must come from 90(?) or 93/94-98, unless you have the '91-'92 forks ('91-'93???) which use the 17mm axle, then you need I believe the '90-'92 ('90-'93?) wheel?
I hope this helps someone! No stem swap required when going this route!
EDIT - I feel the need to add here that you should look up the exact model and year on the Race Tech Product Search to see what original spring rate came in these forks you're buying, as the aftermarket options for the 43mm forks are very slim. The stock springs were in the .38kg/mm-.40kg/mm range, which is great for a 145-175lb woods rider. These forks are not substantial enough to support a 195lb aggressive woods rider/racer however, so I wouldn't really consider them if you are heavier (there is one heavier spring rate available from Race Tech, but you should measure the springs and ask Cannon Racecraft if they have one that will fit them or browse their catalog for that year make model. They may not list back that far, but may in fact have something to fit.
Also, the forks are likely to have the original springs in them still, but also, that's just not guaranteed if it has been resprung sometime in it's 30 year lifespan...
