A full service detail should be able to do it. If you want to try it yourself, just use a mix of diluted Simple Green and a small amount of dishsoap. This should clean up just about anything in the engine bay. Don't use a direct spray of water to rinse it off though. Use a light mist if possible or a low pressure shower. Some people have suggested covering the distributor, spark plug holes, and other important wiring/electrical components to keep water away from them.