Josh,
I saw the Bachmann model at Chicago a couple of weeks ago, and dimensionally it looks like it was based on the Mexican 30" gauge 2-8-0 that is running on the Alder Gulch Rwy in Montana. The firebox sticks out behind the cab, as did the Mexican engines. They have done some strange things with it though, as it has a coal tender, but it has the oil burner's stack firing light!
My guess is they got some Baldwin export 2-8-0 plans and took the details from an existing tourist line engine, possibly one of the Loop engines. It looks like a good starting point for some conversions, but it isn't big enough to make a C-21 out of it.
The model of the Eureka in 1:20.5 scale that Bachmann was showing was beautiful! It is almost enough to push me over into garden railroading!