I wanted to avoid having to modify the gear at all costs, because I would have no idea where to get another if I messed it up.
The adaptor is similar in shape to the shaft of the planetary gearhead (9.5mm shaft, with 2 opposing flats). Also turned a 6.4mm diameter on the end to help get a tight fit. 12mm of 16mm dia shaft is left on the end for motor attachment.
Next, the opposing end was bored with a 6.35mm, 22mm deep hole for the stepping motor shaft. The shaft is held in place with a M3 grub screw. The overall length was around 32mm. But thankfully, the offsets from the motor mounting plate still allowed the gear to line up roughly in the same position as the old one.
The gear itself is currently attached onto the shaft using a press fit. Somehow I produced a slight taper on the adaptor (probably from lack of skill whilst sanding >_<). Exploiting this unintended bonus, I pressed the gear in gently using a vice, being careful not to crack it in case it was brittle.
Had I possessed more forsight, I would've made the 6.4mm shaft bit longer, and included a groove for a C spring clamp. Improvised a solution by tapping a M3 hole in the end, in case I need to use a countersunk screw to hold the gear in.
I don't have a CAD program on my computer at the moment, so I hope my lengthy explanation will suffice
-Wilson-