Well, everyone, thanks for your help. I had tried to place the flywheel
over the key onto the crankshaft (using grease to hold the key to the
groove), and the same thing happened. It started, ran, then sheared. I
have ordered five more keys and will try doing it a few more times. If
you have any more suggestions, let me know. If not, I will try my $10
of keys, and then buy a new tractor.