If you get into taking it apart, I've been there and done that.

Lift the field (outer casing) off the armature - The field is a permanent Magnet so it will try to bring the armature with it off the back plate.

There are often some little thrust washers between the armature and the end bearing so don't loose them

The problem is trying to get the brushes held out while putting everything together

The solution is to just install the armature onto the backplate/brushes so you can get to everything.

Then slide the field down over the armature. WARNING, that permanent magnet will suck the armature up off the brushes so you must hold the armature down while you insert the field down over the armature. The hard part is done with no strings attached to the brushes.

You can handle the rest of it.