Use a degreaser and let it sit for a few minutes before spraying it off. DO NOT USE A PRESSURE WASHER!!!!! It will get into every nook and cranny and possibly mess up the engine.
Be extremely careful when using a degreaser - DO NOT GET IT ON THE PAINT!!!
After you have successfully degreased the engine, spray the engine with no-touch tire foam. It will make the engine bay look brand new.
I use no touch on my inner fender wells and frame and suspension components too - makes them look new without having to re-paint for every show