I like ditch lights on a steamer - so modern and if you squint enough, it's like steam didn't go away...
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I have cut and sliced and diced die cast zinc many times - so long as re-assembly can be done securely and cleanly with epoxy it looks just fine and is very tough.
Modifying die cast can be a pain, though. Zinc makes wonderful models, but it has it's own quirks - sometimes soft when you need hard, and really tough when you need more cooperation.
To echo Mr. Water, brass is easier to work (I've done that too), and soldering is durable in even small spots (epoxy is not - it's great, but needs more surface area). Brass is kind of like the styrene of the metal world.
But - you can combine the 2 materials (I'm doing it on a current project). Get an MTH loco and extend the bunker with brass. Adding the ditch lights should be the about same in either case.
Detail parts are available on eBay; Precision Scale - check them.
If you have not done this sort of work before, start with a smaller, less-critical, less-expensive project of something that you also want. Get your feet wet. Make some mistakes. Better on that cute 0-4-0 than on your 4-8-4. Epoxy works just fine on brass, too. Can't solder zinc.
Mostly, though - do it. Start small - too much at once will lead to frustration.