It could possibly be a bad coil, but you also need to look at the points, condenser, wiring and plug wire & cap. The cap should measure around 5K Ohms, the wire should be clean and shiny on both ends. If the wire is molded into the coil, then don't worry about that end.
If unsure, get a new NGK plug cap, cut 1/4" from the end of the plug wire, and then install the new cap. Sometimes the wire insulation can be stiff, and it makes installing the new cap difficult. If this is the case, push a thin awl or pick 1/2" into the end of the plug wire, and then the cap will screw-on easier.