Double check that timing key
It is easy to break them when tightening the flywheel bolt
I have done it more than once.
If there is the tiniest piece of debris or rust on either taper they do not lock together.
Lapping them in very fine grinding paste is a regular type job down here.
Also the taper must be dry.
If you put anything on it like grease, anti-sieze or even WD 40 it will not lock so the first time the engine cranks the key will shear.
Some times I have needed to resort to stuffing a cylinder with rope so I can get the bolt tight without shearing the key.
In these cases I use a paint pen to draw a line down the flywheel & across one coil arm with the cylinder at TDC so I know it did not shift when tightening the flywheel bolt.
I have also been caught out with debris in the bottom of the bolt hole so was just compressing the debris & not bearing down on the taper.
magneto coils are also handed so they have a right & wrong way to be fitted.