Alright, I'm going to offer a synopsis of the problem. The 9 tooth starter has a slightly wider overall diameter of output gear than the 8 tooth manual starter. The small difference in diameter means it requires more force to engage with the ring gear and that extra diameter needs to be offset somewhere. This is where the shattered nylon gear housing and the difficulty in turning over is significant - that is the effect of the incorrect starter motor being used as it is literally jamming itself to engage. There is another issue waiting to surface which is a bad one - the ring gear getting damaged from excessive wear. Source a used manual starter and test the theory, then decide if you need a new starter motor. Like I said, just a theory...