Chris, This is an older PS-2 5V system. The PS-2 is in the tender, the engine board is a MUX that allows one light signal to control 2 features.
I am confused you now say the headlight and markers work? Before you stated the they did not?
The interior and firebox are on the same circuit (it is a double bulb harness). That can blow. SO if all the other lights work it can be the bulbs or the single FET that controls those lights. The fact the engine is starting to act up also, worries me. There can be a pinched wire, the harness can have an issue, it can be a bad MUX board, smoke fan can be going bad. Is it possible the wire harness to the MUX or another wire was pinched while you were inspecting the engine?
Yes FETs can be replaced. At this point I would recommend taking it to a MTH technician, or you can ship it to me.
The board need to be tested separate from engine to ensure all features work, then we need to figure out why the other faults. G