In most cases, replacing the brushes and cleaning the commutator bar will be enough to make your miter saw run like new. In more severe cases might call for new brushes and a whole new armature.
Worn brushes can also cause the electric brake on your miter saw to stop working.
Nine out of ten times only the brushes need to be replaced. This is good news, as brushes are usually cheap and easy to find with a google search of your miters saws make and model.
Replacing the armature is typically more difficult, and much more expensive than brushes alone.
My recommendation if you need to replace the armature is weigh the cost of replacement part against a brand new miter saw.
An armature, if you can even find a viable replacement, can be over a hundred dollars. It’s your choice if you want to extend its lifespan or spend a couple hundred more for a brand new saw.
Remember, if your armature is at the point where it needs replacing, other parts will also be near the end of their life cycle.
You might find yourself spending another fifty dollars here and forty dollars there and before you know it you could have caught a new saw without any problems.