Are you sure LIONEL offered a postwar Northern Pacific F-3? I don't think they did, though paint and decals are available if you want to model the classic North Coast Limited in Lowey two-tone green with white stripe.
I've done what you plan to do in two respects. First, I've used PW F-3s to repaint (but in road names LIONEL never offered at the time I painted them: GM&O; CB&Q; and Northern Pacific). But for these I used railroad colors intended to match the prototype.
Second, all units received comprehensive mechanical and cosmetic refurbishments.
Good luck with your project. Looking forward to reading more on your efforts.