Using a few threads on here and other sites as reference I installed a WR426F stator and WR400F flywheel on my '99 YZ400F. I have only been able to run it up and down the street a few times, but the mod seems to have worked. Here's what I did:
I go a stator and flywheel of ebay, $99 each, it took a few weeks of watching to get them at that price.
I modified the stator to work with the YZ CDI and to floated a ground so I can run 12v DC in the future.
The first thing to do is find the light pink wire. On the plug it is on the clip side in the left most position
On the stator it connects to the large taped coil, when looking from the top it is on the right side of that coil.
In order for the YZ CDI to work this pink wire needs to go from the plug to a chassis ground, not the stator coil.
I accomplished that by slicing open the sheath about 6" from the plug and cutting the pink wire.
These are all the steps needed to use the WR stator with the YZ. Attach the Yellow lead to a regulator and you can run lights off AC and ground them to the chassis.
I wanted a floating ground that I could use with a 12v DC system. To get a floating ground I did the next few steps.
Clip the pink wire right where it meets the stator coil.
Unsolder the copper coil from the ground lug in the middle of the stator, it is the silver blob next to the upper left black screw.
I then routed the free end of the pink wire behind the blue and brown wires on the opposite side of the coil form which it had been cut. I also slipped a bit of heat shrink over it.
I soldered the pink wire to the end of the coil that I had removed from the ground lug
I covered that joint with the heat shrink that I had placed on the wire.
On the other side of the pink wire that was left loose in the wire harness I spliced on a lead that would be the floating ground. I don't have a pic of that splice, but it was spliced to the left wire shown here.
I feed that wire out the end of the harness and covered the loose end in heat shrink to protect it. It is the orange wire next to the yellow lighting lead.
I then wrapped the harness in heat shrink tape and re-tapped the plug end.
To get the bike running I just plugged it into the YZ harness and grounded that black lead. It runs great, I can't really tell any difference from the YZ stator/flywheel.
When I add the 12v and Lights I'll write that up as well.