The only options you really have, some already covered, remove brushes and thorough inspection, look also for cracking in the carbon itself, had that before although I'm not sure if it's cause sparking. I've dressed off older brushes with a file, perhaps the tips of the carbon get contaminated, again not necessarily would cause sparking, mine have been on the thermocouples on another application...but the contamination can cause problems.
Thorough inspection of the commutator, I've used a hacksaw blade to clean out the undercuts... maybe some electrical cleaner spray to clean off any deposits on the commutator. In the old days i used to use emerytape to clean off commutators on generators. Spin motor by hand as the article states, are the bearings ok ?