You *can* spray the engine with a foaming degreaser when the engine is warm (not hot), let it sit for a bit and then hose it off. Make sure relays / fuse box / air box / battery / alternator and anything else electrical is taped up or covered with plastic.
Then get in there with some rags and All Purpose cleaner and clean everything by hand.
Problem being spraying in the engine bay can be risky. I have done it once but I'll never do it again because I just keep the engine bay clean all the time now anyway. Also the smell from degreasers gets in to the air con system and takes like a week to clear.
Ask Daniel
memberlist.php?mode=viewprofile&u=5301 what he does. He has done a few car refurbs recently and fixed up the engine bay.