Take them to a Autobody shop. Automotive paint is about the highest quality you can get. Top quality urethane will be used by most collision shops. See if they will let you just go back into the paint mixing room and pick some leftover colorcoat (basecoat) on the shelf which will be tossed out soon anyway. Most everything here (USA) is clearcoated so your only expense should be a little labor and clear coat.
Sand the parts thoroughly with 600 grit wet or dry sandpaper
Have the shop spray one coat of a good 2K (two component) (epoxy) non sanding primer/sealer
Then spray 3 to 4 coats of the base colorcoat until hiding is achieved
Then apply one light coat followed by a medium second coat of urethane clear coat. The trick here is to keep the film (mil) thickness down to a minimum level. You don't want a beginer laying on tons of clear as it will create a mess on these small parts.
If are trying to get close to the original engine color look for silver "pearls" instead of metallic as the flake is much finer. The silver in silver paint usually comes from the flake or pearl and not pigment.
Powdercoating is an option too ( and less expensive) but film thickness can be a problem also with small parts.
You can always just "rattle can" them as the parts in question take a beating anyway.