I think there's an issue with the tooling, which means they'd have to start from scratch for the engine (although the tender still kicks around).
Washington's Axe? [Ship of Theseus]
The USRA 0-6-0, 2-6-2 and 4-8-4 Northern may look like the same models from the 1980's but they share NO tooling with those models. It went something like this:
Early 80's: original 0-6-0. Frame and shell extend below cab.
82/83: Chassis and cab retooled to fit leading and trailing trucks. Molds modified from original tooling.
2004: Chassis all new (gears, worm, frame, lower profile wheels). Motor and shell carried over. Running gear modified to fit split frame chassis.
2015: Chassis carried over. All new shell, pilot.
2016: All new chassis with new gears, motor, running gear, side rod stampings. Shell modified for smaller motor.
It really is quite amazing how often this stuff gets changed around.