The vacuum cleaner in the garage was making one hell of a stink and producing a lot of sparking - seems that some gunk had become trapped in the motor brushes/commutator, burnt and caused the brushes to start sparking excessively. I stripped the whole thing down and gave it a clean up.
I mounted the motor armature in the 4-jaw chuck and clocked it in as best as I could, touching off the bearing at the end rather than the existing commutator surface. I then used a shear tool to take a tiny amount (0.05mm in diameter) off the commutator surface to clean it up. Shear tools are great for gummy materials like copper - they don't take much material off, but leave a super finish. After turning, I used some 800 grit sandpaper, then cleaned off the burrs left using a razor blade. The commutator was then cleaned with alcohol, compressed air and a toothbrush to remove any little fragments of copper. I also sanded down the ends of the brushes to give a fresh surface since there was plenty of material.
After assembly, it worked much better - still smells a bit, but not as bad, and there's not nearly as much sparking as before. Should be good for another couple of years!
The airflow path is very clever - there's an impeller on the motor shaft, as you might expect, but there's also a fixed set of vanes/air guides which channel the air back down through the motor body itself to keep things cool.