Check the condition of the ring gear on the flywheel for damaged or missing teeth. Depending on engine could be an issue with valve clearances and/or compression release breaking the gear due to kickback.
Is the engine turning over easy? Or does it turn about half way and stop for just a second? That would be (usually) the valves out of adjustment. Cause a lot of stress on the gears.
I've used metal gears on some before. Louder cranking. But they don't wear out as fast.
The problem with them are, they'll wear the starter shaft, that's plastic. Metal and plastic aren't really a good combo.
