Nice. It looks like a fun project for you.
For the horn, try hooking up the horn unit to the other bike to see if it's the horn or the power supply. If it's the horn, try turning the adjustment screw in or out.
If it's not getting power, you may need to take apart the switch and clean the contacts. On the '87 I'm working on, it was a combination of both these things.
It may be oxidized contacts on the kill switch, too. Personally, I've trained myself not to rely on the kill switch and just use the key. That way, there's no chance of running the battery down by leaving the key on.