If you can snag it to get the flywheel in the correct place by some other means other than a key, then go for it.
Once the flywheel is tensioned up the key does nothing, it is kept in the correct place by the interference fit of the tapers.
Do not tell anyone but I have once fabricated a key from a bit of hardwood and it was fine.
The keys are designed to break if undue rotational stress is placed on the flywheel/shaft fit.
It is made of soft metal because if the flywheel does spin it will shear and not damage either shaft or flywheel.
Warning do not try the wooden key. Spend the $4 on a new woodruff key. Love Gerry