There might be a better, more economical way. The OP is contemplating 2-rail, although he has not mentioned N&W on the2-rail forum.
Williams made a credible āJā in 2-rail, and apparently the used price of these beauties is below $400 at the moment. Mine had a badly out-of-balance flywheel and very wide tender truck sideframes, but other than that it fulfills my desire for a āJā and was easily fixed.