good chance that key indeed is part of the solution. It goes in two slots at the same time, one on the axle shaft and the other in the mating wheel hub.. thus making the two turn as if one.
Most likely when you removed that wheel and hub assy to get the tire fixed that square key key quietly fell out at some point and got lost, as it were. re-install the correct key and you should be back in business.
Good luck - let us know results.

Have a helper sit on the seat and put the tractor in gear and try to drive while you observe the end of each axle shaft at the wheel.
If you see one of the axle shafts spinning inside the wheel hub, the one that is spinning is missing its axle key.
The key is shaped like a small bar of metal maybe 1/4" square and about 2" long.
Concentrate on the "key issue" and forget about the magneto.
The only thing you have done that is related to your described symptoms would be the axle key.
Look all around where you removed the wheel to see if you can find the key.
If you transported the wheel in the trunk of a car or the bed of a pick up truck..........go back and see if it fell out in transit.
If it fell out where the tire was fixed there may be a slim chance that the person found it and is saving it for you.
There is also a chance I might win at Powerball.