Hi Norm,
Just make a list and Check the usual things
1 is there a nut holding the flywheel on or a ratchet for the old starter.If the ratchet is just hit a small amount to tighten, the key can break.
2 is the motor electronic ignition or points.
3 did you get measuring calipers to measure the width of the flywheel so the crank nut doesn't bottom
out before the flywheel nut completely tightens as the flywheel key can break that way and also
if the old ratchet nut isn't tight enough.sometimes the tapers are slightly machined differently and
one flywheel will sit slightly lower and this can cause the crank nut not to tighten correctly.
4 are the magnet spacings the same and north south poles in the same position.
5 are the 2 flywheels both cast iron , shouldn't be a problem here as I haven't seen an alloy 10.5 flywheel.
6 has the coil been removed or touched to fit the flywheel. as coils can fail just with temp changes and movement.
7 Ensure that the new flywheel's magnets are strong with a flat screw driver.
I would check the flywheel key hasn't sheered and magnet strength.
I have had engines work fine one day then the next day it wont start even though you are still getting a spark
and after putting another coil on they run again.(same thing with spark plugs)
Also if you just install the coil quickly ,I'd just double check it hasn't been put on upside down if that's possible.
Usually I measure the tapers in both flywheels to make sure they are close enough so there's no problem.
If it still won't run I think we both do the same thing and disconnect the kill switch wire in case of a short.
Cheers
Max.
