Put the new smaller key in, smeared some threadlock around, put the flywheel on and as the nut was being tightened noticed two cracks radiating out from the centre of the flywheel.
The flywheel (marked 53D) from an old Toro has exactly the same OD and I was happily getting ready to bodge up a starter arrangement but noted a big chunk flaked off one side of the slot in the crankshaft taper therefore will be scrapping this motor.