Hi Peter.
Seasons greetings.
Your tapered key in a parallel key way will only “ bite “ at the point of contact.
It would be better to place a packer behind the back of the flywheel and broach/slot a tapered way through the boss.
I remember years ago restoring an EEC gas engine for a customer, this engine had been fitted with taper pins instead of the usual gib head key and way. It was a rather neat way of fixture, I’ve never seen it done on anything since!
Cheers Graham.