Yzf to yz suspension??

Triple clamps, wheel, axle (and nut), bar mount assembly (with rubber inserts).  
Maybe front fender too, or drill new holes if it doesn't line up.

What he said ^^ but I believe the steering stem length is also different so have those swapped out. Maybe Luxon can chime in & confirm 

If you're trying to bolt the four stroke fork to a two stroke, the easiest thing to do is shim the top clamp (needs a 1mm thick shim). Two stroke clamps use a 2mm larger diameter top clamp than four strokes. 

The right thing to do is to swap triple clamps. You'll need the matching four stroke clamps, but you'll need to swap the two stroke stem into them. Yamaha is a nightmare with clamp fitment...  

Not sure on the axle side, but you should be able to take some quick measurements to see if you have both bikes side by side. 

This is a bigger project than you may think.

Most late model Yamaha four stroke forks are longer than a two stroke.  
They also have a larger axle diameter which requires a new hub and bearings or complete wheel.

The YZ125 and YZ250 handle best with the forks pulled up in the clamps a bit already, so it's likely you'll have to run the four stroke forks up against the handlebars or reduce the fork length.

You can shorten the forks by putting a spacer in the cartridge on the rod under the mid valve / rebound piston post.
You'll have to change the spring length or you'll go into substantially increased preload.

You can reduce the preload on the springs by relocating the spring perch clips or running shorter perches or fewer preload spacers.  What works depends on how the forks are currently setup.
Some kit forks have spring tubes depending on who built them...   You may not have room if there aren't already spacers without cutting down the spring tubes.

If you have both sets of forks side by side, compare length before you try to bolt them up and ride.