First, study real engines, not just reference pix. Engines all have nooks and crannies which collect oil sweat, dust and ozone.
Get metalizer or Alclad finishes that match the metals you've seen. Things like Holley carbs, linkages, hard lines look best done this way and have a very slight sheen. Then stain selectively with very thin oils in umber or gray. Create shadows in manifold low spots, spark plug holes and such but be SUBTLE.
Here's a reference shot which illustrates some of this:
For well used engines, tone down chrome by scuffing lightly with 0000 steel wool-not painting. Use thin ink washes to simulate oil sweat and fingerprints around valve covers and distributors. Avoid blue/red/yellow wires and hoses-black is they way they were originally and adds to the distressed effect. Use clear blue to simulate heat on chrome and clear orange and tea stains for gas stains on carbs and intakes. Stainless exhausts should not be bright and must have different tans, grays and browns on hot/cooler areas. Look at the real thing first. Add weld seams. Then add a mist of thinned Future for a slight sheen. Dullcote sprayed on various areas simulates dust and road film very well.
Practice a lot and study the real thing.