To test the armature easily, take it to the oldest starter, generator, and alternator rebuilding place. For a few bux, they can test an armature on a device called a "growler".
Test the field windings for continuity.
If you have a good armature, a rebuild is easy. Turn the armature contacts on a lathe and dress down the insulating material between the individual armature contact bars using a hacksaw blade broken in the middle. Use the broken end. Or you could use a jigsaw blade or saber saw blade held with vice grips.
After you have replaced the brushes, pack the bushings with high temperature grease and wipr away the excess after the armature is fitted to the end plates.
Test it with jumper cables before you install it. Don't let it spin for more than a second or two. Then install it.
When you are starting your bike, don't let the starter grind on for more than a few seconds. They are very powerful motors and they get hot quickly. Heat is the enemy of anything electrical or electronic.
A rebuilt OEM starter is better than a cheap new one. (Chinese)
"Some Cats Got it, Some Cats Ain't"