SteveBless wrote on 06/10/13 at 07:11:53:

Yes except for one more thing. You will need the head assembly as the 400 head does not have the decompression controller/lifter.

This is the 650. The HOLDER, DECOMP CABLE is held in with 2 crews as I have highlighted with red circles.And the SHAFT, DECOMP is highlighted with the single circle.

This is where the decomp shaft would come through if this was a 650 head.
Buy a 650 head and be done with it.
Maybe if your clever you could drill the holes and mount one. I have never done this. You could put the 400 on without the Decomp. But run the risk of burning out your starter.
My decomp is manual and more often than not I don't use it.

Cloudy wrote on 06/10/13 at 16:35:37:

Yes it could be possible. I swapped complete engines and didn't have these problems.

If you only ride in warm weather, if you do not install a Wiseco high compression piston, and if you awlays have a good battery.....you might be able to do without the compression release.  If you hit the starter button and the piston is on the compression stroke and it stops....you have to immediately let off the starter button, then put the bike in 2nd or 3rd gear and pull backwards to get the piston off the compression stroke.....then put it back in neutral and then hit the starter button.  It happens to me about 1 in every 200 starts, and I have the Wiseco piston that increases the compression from 8.5 :1 to 10:1.

To modify your head not only do you have to drill the horizontal hole for the shaft - you have to drill and tap the hole for the bolt that holds the shaft in.  It is the bolt in this photo that is angled on the other side of the valve cover hole.