Anyway, this is still in a proof-of-concept stage, as I'll need to figure out just how I'll put these back together, and what parts I'll need. This is a test, with strips of styrene behind the side to bring it to the right plane. It's just tacked together with Aleene's Tacky Glue right now.
Here it is next to an (as yet) unmodified Rapido car:
You can see on this end that there is a gap I'll need to fill as well. The best approach would be to add a bit of filler on both ends and center the side that way. Overall, though, it looks like this is going to work pretty well. I need to figure out how to do the window glazing. Most likely I'll just have to put something behind the brass car side, so they won't quite be flush with the car side. But the brass side is pretty thin, so I don't think it will be too bad.