I will start with saying I’m not really an NP guy. So the following is my best construction. But it’s pretty close, I think:
Unlike the GN (see 1947 and 1951 Empire Builder), there never was a “new” NCL. The newness kinda dribbed and drabbed in. The 1947 NCL wasn’t fully streamlined. There were typically heavyweight sleepers in the consist, for example. As it got older, it got “smoother”.
But that’s a good thing for people who like the pre-Loewy NCL. Such as yourself. Because most (or maybe all) of the lightweight cars you’ll need for the pre-Loewy train were also in the Loewy train.
The trick’s gonna be deciding which “year” you’re interested in. So you’ll be wanting an appropriate consist list. I would expect that by 1953, the heavyweights were gone, for example.
As an example, consider the consist on July 18, 1948 (page 168, “Northern Pacific Pictorial-Volume Four” by John F. Strauss, Jr.)
It’s all lightweight (streamlined) except there’s two 14 section sleepers, two 10-2-1 sleepers, and a 6-6. All heavyweight cars.
Moving on, there’s a consist for December 11, 1952 (note that that is winter season) (page 12, “Northern Pacific Pictorial-Volume Five” by Strauss) that is all lightweight. And it APPEARS to be made up of cars that would also appear in the early Loewy train.
So, the chances are good that you can have some sort of a not-Loewy painted NCL by simply re-painting Loewy ones. If Walthers didn’t do it for you.
Myself, I’d also be on the lookout for some heavyweight cars.