Just grind the can. Its easy. Push the pinion through a plastic bag, as to keep metal shavings away from the magnets. Turn it on, and grind it down. It really is just a hunk of metal and will be totally fine.

If you are like me and watch Horsepower TV on Saturdays and Sundays, you see people grinding down metal all the time. It is just the way things work when you are trying to make a sweet custom project.