If the external Novi coil has a crack in it and seems to be chalky finish on it it will likely be problematic.
ALL Mobys suffer from a crappy bonding strap between the motor and frame. Original is mounted at the upper motor mount long bolt and goes to the external ignition coil's lower stud. Replace that wire with one that runs from one of the intake studs and goes to the external ignition coil's lower stud.
Mobys also have troubles with the condenser, heat is a byproduct of electrical generation in the magneto, heat is the sworn enemy of condensers, Novi 120 magnetos do not have ventilation in the flywheel. Disconnect the original condenser from the points (Blue wire or Blue with black tracer) and remove the old condenser. Get a mid 1960s Delco 6 cylinder condenser (or similar vintage BoschVW condenser) and mount it on the external coil's lower mounting stud (either condenser comes with a handy mounting bracket included). Splice the pigtail to the grey wire that goes to the external coil.
Clean the points with a metal file and flush with brake cleaner or alcohol (sand paper leaves grit behind that can cause arcing on the points, carb cleaner has a lube in it which will cause misfiring so dont use those) and set the points gap at index to 0,4mm. Set the spark initiation at 1,5mm BTDC.
Do not use a resistor plug because the antiparasite plug cap already has a built in resistor.
You now should have a goodly bright blue snappy spark that is strong enough to be seen in just the shadow of the bike.