Thanks again for the guidance, Roland (and, yes, the
diagram was just where you'd said I'd find it).
Once I saw the starting circuit, I went back to the full diagram, and it hit me-- it was the Amp Gauge! (I mean, maybe it wasn't-- what do I know? But having checked what I'd checked, and thinking about how I'd fried it in the first place, I suppose I had a strong hunch).
Anyway, I jumped the terminals on the gauge, reconnected the battery, turned the key, and heard the delightful sound of that engine starting again. New gauge is on its way.
So glad I found you folks and this site. Thanks again for the help!