One way to approach it is as a modeler - find a replacement (good or not?), adapt one from another loco/brand that looks fairly correct, or make one. These can be made from styrene and/or basswood. Painted and screwed on, you won't be able to tell the difference at a glance, or maybe even close up, depending on your job.
My point is, this is model railroading, and being able to build at least simple things puts you at that much less at the mercy of manufacturers, suppliers, eBay and train shows. Pick your battles, but when you can build it, it works well/looks good, that's a win. Satisfying, too. What's the worse that could happen? You advance your skills and get practice. That "fuel tank" is mostly hidden under the loco and really consists of just basic shapes.
