8N starter spins and won't engage.

To remove the starter, unscrew the bolts out of the block & put a nut on one of them. Otherwise, the starter comes apart. Not fatal, but not fun either. Then, loosen the two bolts holding the oil filter canister to the block, remove the dipstick, and keep the starter close to the block while pushing the front of it down and lifting the back up. Sometimes you have to remove the drain petcock as well.The bendix is behind the flywheel; your job is to get it over the flywheel.

To install the starter, keep it close to the block while pulling the front of it up and pushing the bendix back into the hole. The bendix has to go behind the flywheel; your job is to get it over the flywheel. Grinding out an egg shape at the 2 o'clock position before you put it back will make your life easier.

Lastly, while the starter is off, polish the block & starter mating surfaces w/ sandpaper to insure a good electrical ground between the two, and clean the mating surfaces where the starter's "belly" meets the aluminum endbell. The starter's ground circuit is through that "joint" as well was through the starter endbell-to-flywheel housing joint.

To rule out a damaged section of the ring gear, be SURE the ignition is "OFF", then turn the engine over a bit by tugging on the fan belt, using the crank (if you have one), or rolling the tractor a little ways with it in HIGH gear.

Then, see if the starter will engage. If not, the starter will have to come off. A quick search of this site for "removing starter" will give you the details of that.

You'll have to pull the starter, and have a look at the drive.