[Al Fielder]

There is a device commonly referred to as a bearing removal tool, it is a split plate that bolts together and has a small hole in the center that has a huge countersink so that it slide under a relatively small gap such as the gap between the drill motor and the pinion. The bolts are tightened and then the whole thing is put in a press so that the motor/pinion is supported (hanging from) the tool. Then the drill motor shaft is pressed out of the pinion in much the same way that the tool shown in one of the previous posts works.

I would be reluctant to cut away the gear (pinion) unless you do a thorough job of masking the motor. If the metallic dust gets into the wrong place in your motor, it could trash the motor quickly.

If you choose to press a replacement gear on the shaft of a somewhat similar number of teeth (but different pitch), stay with a .0002-.0005 interference fit (press). If you go much tighter, you may yield the gear and you could easily lose the press i.e the gear now spins on the shaft.

Good luck