Summary

ATTENTION: To make this work, you must read these notes!

All holes were designed such that the machine screws will self-tap and hold directly in the plastic. If this doesn't work for you, you can try increasing your wall thickness. Using the standard toolhead and the stock HIPS profile, the wall thickness will cause these holes to print slightly smaller than the model dimensions. I've printed mine in HIPS and this has been working flawlessly.

You will need the motor, cooling fan, blower fan, limit switches, and wire harness, and other electrical components, from a lulzbot toolhead, and also a very long cable tie to hold the motor. You'll also need two short 2mm machine screws for the cooling fan, and machine screws slightly longer than stock, for the hotend block (I explain this more later). In my case, I disassembled my flexystruder since it had reached its final days and I was replacing with the Flexion extruder.

You will also need electrical insulation between the hotend and the stepper motor, because the auto-leveling sensor wire must connect to the hotend but it can't be connected to the motor housing. Make sure you test using a multimeter, that you have complete electrical insulation between the aluminum hotend base and the motor housing. If you don't have full insulation, the leveling process will behave erratically, often times not being able to probe correctly.

How to Add the Electrical Insulation:
Add nylon washers to both ends of the machine screws that fasten the hotend block to the motor housing. You'll need slightly longer screws than what comes with the Flexion extruder. I'll update this description with the screw lengths later. Also, drill out the holes in the hotend block so you have some space around the machine screw. Wrap the entire length of each screw liberally with teflon tape (this is a good electrical insulator), except for the very tip. You need just enough thread that it goes into the motor housing screw holes without issue. Assemble these longer screws with the teflon and the 4 nylon washers, and you should be ready. Optionally, you can wrap the hotend block in a thick layer of teflon tape to ensure you don't have contact anywhere else.

If you need to modify the design, it's freely available as a public OnShape document:
https://cad.onshape.com/documents/99bd7cf42021b3071d9ed3c2/w/5a8f6b81c37fc2791429a1c4/e/f5b219dc00aec25b5e49f3a0