Scalecrafts' stacks are probably resin cast, not aluminum alloy. Although you can cast low melting temp. metals in most RTV rubber or silicone molds. Alumilite also sells very fine powders that you brush into the molds before pouring resin. The cast piece's surface takes on the look of what you surfaced the mold with. This can be aluminum, bronze, silver, etc. You can paint the molds also, so that the finished piece is already painted.
Scalecraft, are those two piece molds? Is each stack cast in halves and glued together? Beautiful paint work. Love how the exhaust residue gets stronger as we work our way aft.
Here a a couple of pics of some serious resin casting I've recently done.