So I've done some more tinkering and reached a breakthrough... The secret to getting the engines apart and the motor out of the firebox area is the two ashpan pieces on the bottom edge of both sides of the firebox. I eventually figured out that these are not cast solid with the sides of the firebox, and are actually just a firm press fit. Each of the ashpan sides has two pins, and there are two matching holes on the inside of the firebox sides they press in to. They can be gently, but firmly pulled off, and afterwards the lower chassis and motor separate easily. Below is a picture showing one of the ashpan sides in place, and the press fit holes on the other side where the ashpan is removed.
