If I was plug boot you would see "lightning tracks" on the boot it's self. My suggestion other than grounds as suggested previously (best way to check would be ohm meter from grounds of the cdi, coil, and engine case to battery negative post) would be to trace wires to the stator and make sure that there is not anything rubbed through especially where it goes through the case. Also I have had solder on the stator contacts get hot and degrade eventually coming appart especially with cheap stators.
P.s. also check ground of engine case while trying to start if stator shorts internally it will sometimes give alternating ground/positive as engine turns