The Huxley extruder uses a Nema14 motor and smaller gear spacing than a normal Wade/Greg Extruder.
It's also using an M6 hobbed bolt.
I have used the same great SCAD script from Triffid Hunter to make a drop-in replacement for Huxley sized extruder's.
I didn't have a lot of time to optimise the hole spacing perfectly, so it is a little tricky to fit, but well worth the effort. Take your time and get the motor aligned first, fix the motor tightly, then tighten the hobbed bolt onto the large gear, fit washers and bearing and slide both the large motor into the body and at the same time the smaller gear onto the motor shaft, tighten up the smaller gear to the motor shaft.
All the files are up on Thingiverse here
Enjoy perfect printing with Huxley.