If you have trouble masking off small items you can get 'liquid mask' from art shops and model shops. It's latex and paints on. When it's dry it forms a rubber barrier over the part you're protecting. Model shops also sell 'low-tack' masking tape in various sizes. The mask can be suspect with some acrylic paints and can 'colour' plastics but I doubt that it'll harm an engine. When dry, you simply rub it off with your fingers.
