My suggestion would be to "roll your own". First look for brass tubing of the proper diameter. If you can't find that, roll one out of sheet brass. For rivets use Archer rivet decals, an excellent product. PSC should have cleanout plugs, deck braces, whatever you need. Once you have one made that matches the short smokebox, pull off the boiler front, and you're practically finished.
I've never seen the NWSL part offered on e-bay or at a train show, probably difficult to find unless you find someone with a finished short smokebox model who has the long smokebox. If you are planning a detailing project, this would be a good get your feet wet project.
brian b