Thanks for the info! learmoia, I will reply to your PM. I will also try giving Con-Cor a call. The Hudson I have runs well, but without the side rods and valve gear it looks like a steam-shaped diesel running down the tracks!
My original question still stands, but now from an academic point of view. I wonder if 3D printing will make for viable replacement side rods and valve gear, especially for old, discontinued models like the Con-Cor J3a. I'm mainly concerned that they will wear out prematurely.