Sorry, you are right, can not see engagement just the linkage operation. Getting under and looking? maybe. NAPA usually has pretty good parts. It's a reman price. As I listen to it, starter sounds pretty healthy. So it seems like drive gear is not reaching the flywheel sometimes (as others have already said). If you get the NAPA, it may not come with foot switch, so take yours off before turning in core. If you are handy, you can replace the starter drive as posted above. Again sorry for the bad suggestion.
