I don't have a lot of experience with this, but in the few engines that I have cleaned up, I knocked off the big stuff with a putty knife, sprayed it down with one of the cleaners and then powerwashed... sometimes more than one application. Be careful if you're using oven cleaner while the engine is in the vehicle, it can do some nasty things to paint and plastic parts. And sometimes when you get the engine all nice and clean and start it up, it leaks like a sieve, and after 5 minutes of four-letter words, you wished you wouldn't have started cleaning it in the first place.
Lee