I remember now that Mantua made their 4-4-2 Atlantic available with either standard or Belpaire fireboxes. They're not real close to GN's Atlantics, but had Sagami can motors and ran pretty well. It's not exactly like the GN engines, but if you could find one and paint and letter it for GN it wouldn't be too far off. Guess it depends on what exact time frame you're interested in modelling??
http://www.gngoat.org/gn_steam_13.jpg
The MDC 4-4-0 is/was available decorated for GN, it's pretty close to some GN 4-4-0's:
http://www.gngoat.org/gn_steam_5.jpg
http://www.roundhousetrains.com/ProdInfo/RND/450/RND84534-450.jpg
Well you could start with a PRR engine and modify it. Fairly reasonably priced engines are the Bachman K4 4-6-2 or one of the Bowser kits for 4-4-2, 2-8-0, 2-8-2, 4-8-2 or 2-10-0. they also have Belpaire fireboxes and are on Ebay fairly often. Bowser no longer sells them. .
For your further education, you might look for MR issues from the late 1960s through the 1970s for articles on building/modifying locomotive boilers. I seem to remember an article by Bud Sima describing how to build a boiler out of "wood and card."
Sorry that I can't cite "chapter and verse" but the information is out there if and when you feel ready to try it.
Dan
Outside of brass, the only steam engine in your price range is the USRA 2-8-2. These engines were generally assigned to the eastern end of the GN, running on trains between the Twin Cities and Duluth/Superior, and between Superior WI and the Mesabi Iron Range.
BLI or PCM came out with a brass GN 4-8-4 maybe 6-7 years ago that sold for around $500 IIRC. They announced a cheaper version was going to be made, but that was 4-5 years ago and so far nothing has come of it.
GN did have a few engines that didn't have Belpaire fireboxes, so you might be able to create some stand-ins. If you took an undec Spectrum USRA heavy 4-8-2 and added a Spectrum Vanderbilt tender, and painted and lettered it for GN, you'd be somewhat close to some of GN's Mountains.
GN was an early user of diesel power, especially for freight trains and switching. They bought diesel switchers in the late 1930's and FT's in 1941-42 (some of which were used in both freight and passenger service), so you could set your layout as far back as 1941 and use a mix of steam (one or two of the USRA Mikes), A-B sets of FTs in green and orange, and black VO-1000 and NW-2 diesel switchers.
I do not have a specific model to suggest to you but you might want to explore the Hornby and other British models of steam because some English engines had the Belpaire firebox. If the basic boiler can be acquired a GN style cab and other fittings could be a modification.
Dave Nelson
Thank You.
I, too, model the GN. There is, unfortunately, very little available in mass produced steam locomotives that are good Great Northern models. (I am assuming you are modeling in HO scale.) I have been forced-in the interests of accurate appearing GN steam-to base my roster on brass. That said, there are a few models that can be used fairly accurately for the GN. One is the Broadway Limited USRA heavy 2-8-2. The GN had 9 of these in the O-3 class, numbered 3200-3208. But, as for mass-produced Belpaire firebox equipped locomotives, it seems that only Pennsy locos have been made with any regularity. They just don't look anything like a GN loco at all, unfortunately-even with the Belpaire firebox.
I model the Great Northern Railway, and I'm having a problem with finding steam engines, less than $300, that have a Belpaire style firebox. I'm new to the hobby and I don't think that I'm ready yet to cut apart my models to modify them. Does anyone know of models out there that have that type of firebox?