Well, I figured out why I couldn't get it all the way back together. I found an old scan of the "General Instructions" sheet that came with the gun. In order to re-insert the hammer into the receiver, you have to pull the trigger to pull the sear out of the way (which I was trying)
The thing is, I can't pull the trigger far enough back to cause the sear to break and release the hammer. This is why I couldn't figure the action out because the hammer wouldn't release. I had to remove the receiver from the stock to get the thing back together. Guess I'm taking this to a gunsmith to have a look at. The whole mechanism is stupid simple, though. Don't want to risk messing up such a cool old gun, though. I've got enough experience working on airgun triggers but with something I don't know if I can get parts for, I'd rather leave it to someone who knows better.