Many years since I have had to remove a tapered gib key, not many used nowadays.
As you said, we used to use an extended drill and tap, screw in a bit of threaded bar and jack it out with nuts and spacers. On many occasions, when there was still a lot of key in there, the thread would strip, then we would drill right thru, with the largest drill possible, to allow what was left of the key to collapse in size due to the taper pressure, and hopefully the flywheel would come off. Never had to go any further than that.
If it is only the last third in there, it should come out very easily, as that part would have very little, if any, of the taper contributing to the locking action.
Hope it helps with your problem.
John