Yeah the original wiring diagram isn't very clear. I know that Mel posted a thread about adding turn signals on a 78, and had a color wiring diagram. Pretty sure the colors are the same because Mel used a 1980 wiring harness. It's possible that your wiring harness is damaged somewhere, maybe, maybe not.
As for the air filter, if you want a new stock one, look on eBay or cut one out of a sponge and soak in motor oil. If you want an aftermarket one (seems like you do) you can get some small air filter from treats. I think the carb intake diameter is 24mm, about an inch.