Daniel
I have built two R1's from Bachmann products, one from a Bachmann Northern and one from a N&W J Class. Both required significant surgery to the ATSF boiler and cab but result in pretty credible representations of the Baldwin Loco. There was a family resemblance among the prototypes too. The Northern version has the wrong type of drivers. You can get away with making an R1D if you can locate a centipede tender. Otherwise I just live with it. The J Class makes a pretty good R1A-C although the drivers are a bit small. The wheelbase is close. The former J Class is a sweet runner. I used Minitrix Delta trucks for my locos and they worked well.
For your reference, I wrote an article about the Northern conversion in the BMRRHS news letter a few years ago.
Bruce