The woodruff key is only for getting the flywheel in the correct position it does not hold any load. If the flywheel rotates (which it should not) and breaks the key then it only affects timing not the stength of the spark.
I always run a wire directly from the stator backing plate to the coil earth to eliminate the possiblity of a bad earth. You can run a temporary earth wire as described to test the system.
Almost broken wires also create high resistance paths which weaken the spark. Plug gap should be only 0.55mm. I have often found that old crimp connectors look OK but once you cut them off they are corroded internally which also create high resistance
Grab a known good plug and grind the earth electrode off. The Motoplat should be able to create a spark between the centre electrode and the spark plug body when the body is grounded. If it does not then the Motoplate need attention.
Also another unusual problem with Motoplats is that if the bike sits for a long time (years) and depending on where the flywheel stops it can magnetise the stator coils and cause no or very week spark yet after a period of use the spark returns. Ross I can feel you rolling your eyes at this point but TRUE STORY
