Re: Welding ring gear
Never seen a ring gear welded on but don't see why it wouldn't work. Save tearing the tractor in half to replace the flywheel and ring gear again. If you just put some tack welds on it thru the starter mount hole you should be OK. I sure wouldn't get carried away with the welding and would tack one side, turn it 180 degrees, tack again and then turn 90 degrees. You get the idea. Four tacks about an inch long should do it.
Since the crank is bolted solid to the flywheel you should be able to ground the welder to the front of the crank and avoid any potential problems of arcing the bearing surfaces.
Worse case you end up pulling the flywheel again and grinding off the tack welds.
Just my opinion, never done it, hope I never have too.
A gun and a parachute are much the same. When you need one nothing else will really suffice.