Hey, I keep forgetting to tell you that I got myself like 10 sheets of this rubber-plastic stuff, it's kind of like linoleum, but it's working pretty well as a coroplast replacement, and it's very flexible. Let me know if you want any, I'm going to try do to a belly pan for my car, but there should be plenty left over.
That underbody picture definitely makes it look bad, and like a lot of work. But, it looks like most Metro's rust out anyway, so it'd probably be hard finding a shell that wasn't just as bad as yours. I would say to go for fixing it, unless you come across a decent body, for cheap.
Or, throw the engine/tranny into your civic