The front end of a YZ426 will also fit up nicely to the DR frame. There are three major challenges with converting to usd forks; the triple, front axle and brake caliper. And three less demanding items; speedo, headlight and ignition switch.
Surprisingly the DR triple stem is larger in diameter than the YZ (30mm vs 28mm), a problem easily solved by leaving the stock DR races in the frame and using the YZ bearings on the new triple. The YZ triple is a little longer than the DR so you will also have to come up with a spacer of about 12mm to get everything to fit snugly.
The DR uses a smallish 17mm front axle while the YZ and many other dirt oriented bikes use a 20mm axle. The easiest solution is to simply mount up a matching front wheel to the forks you have chosen.
Last item on the list is the brake caliper. Fortunately the DR uses a common design that will bolt right up to the YZ and I believe other models as well. The stock YZ rotor is 250mm vs the 290mm of the DR. An aftermarket EBC oversize rotor with mounting bracket measuring 280mm is readily available.
Most of the usd forks have more travel (11"-12" vs 10.2") than the DR so the front end may be a little long after mounting everything up. Easily solved by sliding the forks slightly above the top triple and/or when rebuilding the shocks slimming down the bushings to lose a little height.
Picture below of '01 YZ426 front end on my '03 DR. Since I was planning to convert from stock speedo to vapor no issues there. And rather than try to mount the stock headlight that I never liked anyway now is the time to draw up a plan for something new.