Metal protectant for engine parts?
Any external exposed engine parts that are steel or have iron in them probably need to be painted with a high temperature engine type paint. (You can get it in a convenient spray can at the auto parts store). 

Anything that is going to get hot from the engine operating you do not want to use an oil or other petroleum based compound for a coating. A clear varnish that is made specifically for metal parts would be suitable. WD-40 --- Never.