Look on the back of the genny - single spade terminal=6volt, black plastic with two terminals=6/12volt.
6 volt will run with no accessories attached, you just need the gray wire from the genny to the external coil. but you better fix the decompressor or you have to shut her down by pulling the plug wire.
6/12volt with three coils inside the genny will run with accessories disconnected
6/12volt with two coils inside the genny needs to have an accessory attached to the 12 volt or that terminal grounded because it is the other end of the winding for the ignition coil.
Is the spark robust and snappy or yellowy and weak? Might need to change out the condenser, Get a 1960s GM 6 cylinder condenser and mount it beside the external coil. Disconnect the old one inside the genny and connect the pigtail from the new one to the gray wire.
Make sure you have a good bonding strap between the external coil and one of the intake studs.