Here's a screen grab of the CAD. I changed the color of the parts based on material. The main structure of the cab will be die-cast, as will the loco frame/gearbox, wheels, steam chest, and the tender truck components.
The loco boiler and the tender body will be brass.
The boiler couldn't be round all the way under like in the photos posted above. There are simply too many features to fit in this engine for all of that. The main PCB is large, there's a dual smoke unit for the stack and whistle, a motor w/ encoder, a die-cast backdrivable gearbox, and more. Not to mention, 90 is a small engine despite being a 2-10-0.
Hope this answers some questions.