I don't see how you can solder without burning the paint. My brother has a 402 with similar seams open. His is in otherwise excellent condition. He choose to leave it as it.
I have experimented with Zymol polish on some inexpensive metal trains. I don't think I'd use it on your loco because I don't think all the dirt / discoloration will come off, so to me, it wouldn't look right being shiny.