To remove the shafts gear, would you need a puller like for a pully and bearings?
I've never removed or replaced the bearings and cog on the end of the armature, but they look the same (only smaller) as the bearings fitted on the rear axle. I had those removed and replaced by my mate who works in a local garage using a proper puller. I'm guessing you'd need something similar for the armature.
Judging by the number of armatures that have snapped, I'd be concerned that fitting new cog and bearings might also cause the same damage.