Don T, if I remember correctly a weak spark can be caused by point gap set too high not allowing the coil field to fully collapse. Since you have a few other Cubs in your stable, I would consider also taking the coil, spark plug wire, and plug to another Cub and see if it will work there. If not you can then substitute each of the 3 items from the running unit until you find the bad part, although you may have already done this and I missed reading it.
As a thought, has anyone ever used a dwell meter on a single cylinder, I personally think this is a much better way to set timing but have never tried it on a small engine. In fact I don't know the last time I used mine, probably on the VW Rabbit my daughter had when she was 17 (She's now 41).