Starter motor failure is fairly common, not many on here with high mileage starters. ring gear i have never heard of problems. I would have a look at the starter first the heat in that VEE under the plenum does not do them any good, and as mentioned the water hoses under there perish with the heat and the water leaks take out the starter and the knock sensors. removal of the plenum is not uncommon and most people when in there replace the hoses for silicon high temp ones, replace the starter with another brand (i used Bosch) replace the knock sensors and also put a new gasket on the plenum under cover and replace the screws with plenty of loctite. But if you really want to remove the gearbox feel free!