Jared, that engine is fitted with pull start as well as electric start. If it were mine I wouldn't be worrying about the electric start. I'd just be using the pull start.
I'm not a fan of Norm's repair suggestions for the wheel. If it is to be repaired it needs to be done properly. Your a machinist, what do you think of the idea of trying to cut a new keyway 180 degrees from the current one? If it were mine I'd try turning up a plug that fits neatly on the parallel section of the axle hole and run a drill down on a drill press to start the keyway then neatly hand file to finish or even carefully scrape the keyway out on a lathe. Would be a fiddle job but I reckon it would work.
Can't help with the sprocket as I used both my spares on the zero turn. My own Tractor 8 is fitted with a generic 10 tooth sprocket bought from an engineering supply place years before I had a parts collection.
