I am going to take a stab at this one.. I have no idea of what I am talking about.. so ride along with me.. so you can't get the starter to turn the eng to gen over.. it has a starter on it.. inside the starter unit.. a long time ago.. the starter had brushes.. and they still do.. but they were made to be replaced.. the starter brushes.. there are 2.. you will see 2 large.. I mean large Phillips head.. the brushes are under there.. Now don't try to use a standard # 2 Phillips head.. you must use an impact driver .. not impact wrench.. the old fashioned one I have in bottom of tool box.. you use the correct head and hit it with a hammer..
So.. if I understand you.. the problem is with the starter.. and you feel that the starter needs new brushes.. ok.. but because these starters are nearly bullet proof. I would check a few things 1st.. how many hours on gen ? Next use another vehicle to jump battery to gen.. leave vehicle jumping running.. and try that.. next.. use set of jumper cables to rule out bad ground.. so to do that.. put any end on beefy part of gen eng.. and the other end on beefy part of frame.. then try..
Next.. if certain you feel it's the starter.. take to a/p store and see if they can test.. or use your jumper cables and test it yourself.. put them on battery and put positive on starter lug.. and just touch starter with negative.. use all safety precautions.. it will jump out of your hands.. so stand on it..
Now.. all that.. these old starter were made to be rebuilt.. they are old and reliable.. made in USA. So most big electric stores can do it for you or know someone who can..
So I am believing.. maybe wrong.. that you have an independent starter motor.. and we are not talking about the inside the armature section of the gen.. am I close? Did I just miss it ?
Good luck and keep us posted
2000 southwind storm, workhorse custom chassis with 7.4l vortec