Here is a photo of what I came up with for my Z axis on my Enco mill/drill.
The large drive sprocket is an interference fit to the hollow angular contact bearing shaft. That shaft has internal threads to attach the ballnut to.
I used the motor mount plate that came with the motor. Everything fit ok, but two of the spacers were too close to the belt and I had to turn them down a bit.
The whole works fits into the spindle housing (I only had to grind away a little of the casting). I plan on making a simple part to mate the ballscrew shaft to the stock spindle snout yoke.
The motor has an E4 encoder from US Digital. I was impressed with how small these encoders are and how easy they are to mount.
Jim