I use Floquil's "Foundation" as a primer. Its a light tan in color and allows an even surface for the colors to adhere to. Another advantage is since its what's called a "neutral" color, and of the more difficult colors to get good coverage with, like red, green, (slight color shift) and yellow, will go on without any extra coats being needed.
Whatever method you use to paint it, and whatever brand of paint you use, you do need to bake the engine in a low temp oven, (150-200`) for at least an hour, (and up to 4 hours,) after its painted. But do this only if the engine has been completely taken apart to paint.