You can put a WR transmission in a YZ if you are obsessed with weight.
Mine weighs 266 wet (3.2gal)
3.2 gal x 6.35 pounds per gallon (91 octane) = 20.32 pounds of fuel.
266 - 20.32 = 246 pounds, dry.
Right now my WR weighs 244 pounds and I have about 8 more pounds of weight reductions planned.
By the time you add the necessities to a YZ you can get a bike of the same weight from a WR by removing all the unnecessary stuff. After all, the WR450 is a YZ250F with an ex, detuned YZ450F engine in it.
If you wanted, you could literally buy a YZ250F and put the parts from it on the WR450 chassis. And some of the things that weigh more on a WR450F are a good thing, like larger radiators for example. Another example is that the valve cover breather connects into the air box rather than being open on the frame where it could draw in water during a deep creek crossing. To say nothing of having EFI and an alternator for powering high output lights if you need them.
Some of the things you get with WR450 that you don't get with the YZ is the wide ratio transmission, an engine tuned for woods riding and suspension that is tuned for that as well. I don't know about you, but I would rather lop weight off a WR than split the cases of a YZ to install WR gearsets and trying to figure out how to detune the YZ450F and make it as docile and tractable as the WR engine is, to say nothing of the suspension being a mile away from what is needed for woods.
And then there is the issue of the frame. The YZ450F frame is larger, heavier and slower reacting than the YZ250F frame, which the WR450F is based on.