There are 3 possibilities I can think of. Brushes, the cheapest and easiest fix. Speed control, usually too expensive to replace but can be bypassed usually. And 3 the armature, usually a throwaway if that's it. Anytime I had problems with the armature I couldn't get it to start in certain positions but by rotating it I sometimes could but at reduced rpm and power.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.