I’m not sure about painting black, I think it comes out a bit dark. Maybe a really deep grey?
You have said Main trunk, there were 2, so if you can be a bit more specific we might be able to do better.
The photo I saw showed a C class loco.
Regardless of its identity it should really just be left in the ground. Therse 2 C’s preserved (Silverstream and westport) plus 7 D’s surviving (2 in running order). I would think spending money on something rarer (ie WMR no9) would meak more sense from a historical point of view.