My Moby sense is saying if you had a bright blue snappy spark before then it might be the master switch from hell messing up your day - disconnect the grey wire going unto the accessory harness from the one going from magneto to external coil.
Might be the stupid DOT compliance as well so try disconnecting the 6v wire fromt he back of the magneto and make a jumper wire to the engine.
Then check for spark again.
Could also be a shyte connection between that crappy red spark plug wire and the spark plug or the coil.
If the spark was yellowy and weak before then it is the condenser crapping out, which you should replace anyhow, or the bonding strap (that has already been mentioned).