one big problem is the ground strap between engine and frame. those HT coilds arent notoriously reliable. also cleaning points is the most common fix, espec for weak spark. condenser is another pretty common one.
check out wiring diagrams in wiki here or on projectmopemanual.org
Basically wanna check power each step along the way. it could be anything but if you start at the power outta the stator, if none there you know its stator. check power into coil (disconnect kill switches and check) and then check power outta the coil, could be the plug wire or the boot or the plug itself