Any idea how to achieve the grayish-white hue of most of the engines' faces?

I'd suggest alcohol to remove the black and acetone or some other solvent for the correction fluid. Just clean the faces if needed first and make sure not to wipe the detail off.

The alcohol solution was, sure enough, what I did to clean a black stain off the bottom of my Oliver's face. It (more or less) worked!

I'll post a few pictures of some of the modification faults I'd like to fix very soon...