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

If you're not too afraid of it, you could try repainting the whole face. You would need to take the face off of the body shell, then wash it with soapy water. A color that I've found seems to match the faces in the TV series (slightly darker than the TOMY faces) is Vallejo Sky Grey. You would need to coat the faces with primer before painting, though. A very fine artist's pen works for the eyebrows.