I've had the best results with a small foam brush, and model paint. Wet the brush on the flat side and lay that on the letters, it may take a few tries to get it right, but it puts on a pretty even coat once you get the hang of it.
If they are recessed, you can also use the sponge brush to fill the letters and then wipe off the excess. Obviously, this works best if the part can be laid horizontally so the paint won't run out.
This idea works well on stuff like your valve cover too. Fill the brush up heavy to do the indented parts, wipe off the excess, let it all dry, and then use the brush with a light amount of paint to do the raised portions in a contrasting color.
Good luck
