I think what they are aiming at is creating that muck over metal look that characterises most engines after a fairly short period of time.
Best bet take a look at some pictures of the real thing and see how they look. Most air cooled engines start out metal coloured, and then accumulate dirt in the nooks, crannies and cooling vanes, so perhaps a base coat of black, followed up by dry brush of aluminium or steel might give the right effect.
Karl