The Red Oxide is simple - just add a touch of brown, with a tiny touch of black, to red, to make a dark red 'rust' colour.
The simplest way to make the plate on the tank is to punch a small disc of very thin plastic, or even paper, and add it to the top of the tanks. If possible, a hole can be only countersunk, for the disc to fit into, but this isn't that important.
Note that the disc itself was virtually flush, with a rim standing just proud, and looked rather like the filler cap, as shown below. It could also be made by adding a small sliver of suitably sized plastic tube, then sanding it down, as I did for a filler on the 'flat' tank on my P-47. When the 'plumbing' is attached, and being partly hidden under the wing, it would look acceptable enough.