the first thing i would do is try a new plug. coils rarely ever go bad. if there is continuity, its a fair bet that its good. the wire or the boot on the plug wire may be bad. usually ignition problems are caused by;
1- bad spark plug
2- dirty or misadjusted points
3- weak condensor
4- bad wiring
5- bad kill switch
6- blown tail light or brake light bulb.
rusty magnets and pick ups on the charging coils can impede spark too. you need to look everything over closely to determine whats wrong. 9 times out of 10 its something simple. if your boke has a battery, check the fuse.