I had a Polaris Sportsman 500 in the shop with electric starter problems. It had a few corroded wires and loose connections between the battery and the starter solenoid and wore out brushes on the starter motor.
ATVs around here get used like automobiles in suburban America. People make lots of short trips around town. The engine gets started and stopped frequently. This leads to a lot of wear and tear on starters and the electrical starting system.
I change these often enough that I keep a few spare brush sets on hand. For some vehicles I have the complete brush holder assembly and for less popular models I have a few universal brushes that I can solder onto the original holder. The universal brushes are cheaper, but the extra time to solder them on cancels out any savings.