Some assembly required.

Adapting Suzuki GT 750 forks (or any other Japanese fork with 35mm stanchion tubes) to fit a Royal Enfield.

Step #1.
Cut a thread at the top of the fork to allow it to screw into the threaded hole in the upper fork crown on the R-E.
This is a British Cycle thread, 26 threads per inch, 60 degree angle. 

 Here's one of the threaded holes in the upper fork crown which the stanchion tubes thread into.

 You need a short section at the top which is cut back to the root diameter of the thread.

Step #2
Modify the hex plugs at the top of the Suzuki stanchion tubes so they fit flush. This was complicated by the fact that the plugs are hollow, and cutting off the hex section at the end exposes the hollow. We worked around this by drilling the hole and tapping it with a 1/2 inch pipe thread, which we then filled with a hex socket pipe plug.
The bonus to this is that it will allow us to unscrew the stanchion the same way as the original Enfield forks, using a large allen wrench.
Here are the plugs as modified.

 Here are a couple of shots of the fork crown with the Suzuki stanchion installed for a trial fit.

 

This view shows the top of the fork through the little hole that allows the fork to be unscrewed with an allen wrench.

Step #3, figure out how to hook up the plumbing for the dual disk brakes. When I bought these forks back around 1990, I didn't know that the early GT750's came with an excellent four-leading shoe drum brake, and by the time I'd learned about that, all the four leading shoe drum brakes had been snapped up by people customizing English motorcycles.
To adapt the plumbing from the Suzuki forks, I made up a little bracket to hold the "splitter" that routes the hose from the master cylinder/brake lever down to the dual disks.
The shape of the bracket is dictated by the fact that it has to fit directly under the  friction damper for the steering. Carefully cut out to just clear the damper. 

 Here's how it sits on the fork.

 Step #4. The Suzuki fork legs are spaced 3/16 of an inch  further apart than the Enfield fork legs, so to use the Suzuki forks in an Enfield triple clamp, you have to figure out some way to split the difference between the two fork legs.
We made up a 3/32 inch washer that pulls the wheel over that distance, and re-centers it. The brake disks just clear the fork sliders, and since the calipers are floating, no modification of their mounting was required.
Here's the axle  and  washer.  I used two right side axle spacers because the one on the left side is the speedometer drive. The washer wasn't really necessary, but it makes it easier to get the wheel centered in the forks.

Step #5 (optional), you won't fool any British motorcycle enthusiasts, but your Enfield will look pretty cool if you make up some little badges to replace the pieces of tape stuck to the brake calipers that say "Suzuki" on them.

Here's a shot of the forks assembled and ready to go.

Forgot to mention the gaiters! They're Triumph gaiters with a flange on the inside bottom edge, which engages a groove in the fork slider. We had to cut a suitable groove in the top of the Suzuki fork leg.
I also used part of the tubes from the top of the Suzuki forks that hold the headlamp mounts to give me something to put the tops of the gaiters over. It attaches the same way as the standard Enfield upper fork covers, using little tabs which are held by the bottom fork crown pinch bolts.