Brushed motor cleaning
Unless it's a particularly unusual motor, given the relatively low cost of these, I might be inclined to replace it with a fresh unit. The cost, time, and materials used to clean out and service the compromised unit may not be worth the effort.
It would be annoying to have a "cleaned" motor fail underway several outings later.
"Good judgment comes from experience, and a lot of that comes from bad judgment." Will Rogers.