I have indeed used CIM motors on the original Kaizen axe, Azriel 0.9's axe and still in Jibril's drive system. They are quite robust little motors but they aren't especially torquey. They do indeed have two mounting holes (10-32 imperial size screws) on the face.

You can see the axe mechanism on Azriel 0.9 from 6:30 as below (from RL Grantham 2019, apologies for the dodgy camera work):



As far as gearing goes, yeah, I use spur gears on them. The one in Azriel 0.9 as above is a 3:1 MOD 1 single stage setup (and I wonder why I couldn't self right... :P) and looks like the below:

axe.jpg

The trouble with single stage transmissions is that although simple, you do end up not always being able to reach the desired ratio and, especially in the FW class, weight of the output gear is a factor. One of the big 127T gears I use in Jibril is 2.1kg and I use two of them. However, if you were to go with this setup, 1x 12T 8mm bore MOD 1.25 gear and 1x 120T say 12 or 15mm bore MOD 1.25 gear will achieve 10:1 here. Just make sure it is lightened.

As far as attaching gears to shafts go, on the pinion side, you could get away with drilling a slot into the keyway for a grub screw to sit in but this depends on how heavy the swinging mass is so I'd recommend drilling the shaft and bolting it. Same for the output gear if you can reach it with a drill but if not, it'd be either a keyway cut or welding the gear on (I am currently going through that challenge with Jibril as we speak). Some lockthread wouldn't go amiss either, just ensure it has time to set fully.

Drilling gears to attach shafts and drilling shafts to attach gears with isn't hard but is time consuming. Slow as you can, plenty of drilling lubrication and it'll work. I would recommend a drill press if you can get access to one to ensure a vertical drill hole.