It's from an HPI Nitro Star .12R SS.
It doesn't look like nickel plating. I own a couple nickel-plated items and it looks shinier than this when polished, and when tarnished it has a distinctly brown color whereas this looks grey with the slightest hint of purple. (my camera didn't do a good job of showing that.) Also, I have the engine and a spare cylinder for it, and they both look exactly the same, which says to me that the current color is not the result of tarnish, because if it were there would be some amount of variation between the two.
That's not to say it isn't nickel-plated, it just doesn't look like I would expect nickel-plating to look.