Replace the motor brushes. I’ve fixed Many tools that way. I hope the commutator isn’t too bad. Most people don’t know to look at the brushes / commutator arcing on all brush motor tools. If it’s a constant low level blue - all’s good. As yellow then white start appearing and the size of the flashes get bigger - replace brushes. When you get the new set, compare lengths. The longer it’s run with yellow white arcing getting worse, that’s damaging the commutator.