If it's in your ability, I would pull the starter off, it's only 2 bolts, pull it apart and clean and regrease every thing. Put in some new brushes, clean up the armature, clean up the push button contacts on top, and try again. Sometimes all that gunk, dust, and dried up grease can get into everything and slow it down. If that doesn't do it, your local auto parts store should be able to get a rebuilt starter, and I don't think they are that much $. 12v battery shouldn't hurt the starter though.
