Changing the end prong shouldn't be an issue if it broke. Did you get a reading before the plug broke? Not sure of resistance values, maybe a quick google search can give you a guide, I remember finding a good video on YouTube a while ago that explained how to check resistance and how to charge and check a condenser with a multimeter. As far as I'm aware the only thing that can go wrong is a broken connection a short or dead condensor. also check to see if the points are dirty and need cleaning
Also one of these spark plug testers make life alot easier when checking for spark, you connect it inline with the sparkplug and the coil wire and you don't have to worry about the sparkplug moving off the engine when cranking. Plus it's easy to read when the globe lights up if you get spark.
