If your going to leave the engine cover off permanently then you can take the plenum lid off (aka top of engine) via the allen key bolts and polish it with an orbital polisher and metal polish. Sand it lightly first and then use the polisher - you could do it by hand but it would take a while.
Otherwise if your going to put the engine cover back on, just leave it, it's only going to get dirty again...and you can't see it.