Do you have access to any Testor's paints?
There are two "stock" options for engine paint, "GM/Ford Engine Blue" (a kind of an industrial blue) and "Pontiac Engine Blue" (more of a silver with a small amount of blue and not really accurate for that model year). This is if you're aiming for total accuracy.
Some other options would be a medium metallic blue or any kind of medium blue if you're looking for something mildly custom or else shoot the moon for a completely custom look.
For example, if you wanted something that makes it look like the engine was totally rebuilt and repainted, you could use what-ever color you liked.
