BBK,
It really depends. If you are building a 1000 point show car then you will need to pull the engine and then take off the accessories, clean and then spray with the proper amount of industrial grade paint.
If you are only trying to preserve the engine and retard corrosion. Then you can degrease, power wash the engine at the local car wash (protecting the ignition parts with plastic bags, duct tape, etc. so you can drive it away), take a can of engine paint and a few various sized fine bristle brushes and dab some on. A slow drying enamel works best at the brush marks will be minimal.
The there is what I call the "show car circuit" car. You meticulously clean, grind,file, fill, smooth then engine (out of the car of course) until the surface rivals the front fender and then prime, smooth again and top coat with a color coat followed by a few coats of clear.
Or if you want it to take less time than option 2 cover the firewall, inner fenders and frame parts with paper and get a couple of spray cans of engine paint and carefully spray the cleaned and degreased casting. Have a rag moistened with thinner to carefully wipe off some of the over spray on the places that were not taped.
The non-show philosophy is if you can't see it it may not need to be painted and the quality of the paint surface is not that important.
