Here's a process I used on my Mustang GT/CS that I learned from someone in the Mustang community. It worked great for me and everyone who tried it. Had the engine compartment looking almost showroom new:
1.) With the engine good and hot, cover the critical components (alternator, fuse box, etc) with aluminum foil. Proceed to spray down everything under the hood with a 50/50 mix of Simple Green and water.
2.) Close the hood and allow the spray and the heat to essentially steam the engine compartment for a good 10-15 minutes
3.) Hose off everything you sprayed, taking caution around those parts you covered with the foil
4.) Assuming everything appears to be cleaned of dirt and dust, spray (insert brand name here. I used Turtle Wax F21) tire foam on the black plastics such as the engine cover, air filter box and intake tube, etc, and allow it to do its thing for 5-10 minutes.
5.) Hose off and remove the foil. Everything should be nice and clean and the black plastic parts should be black, not dirty black/gray looking.
If it doesn't look clean, you may either need to repeat steps 1 & 2 if there's still caked on dirt/dust, or repeat steps 4 & 5 if everything looks clean, but not new/black enough.
I did this several times with my Mustang and it worked each time, with no ill effects on the engine. And I had a CAI on it too. The engine bay looked brand new. The key is to make sure the engine compartment is good and hot (I did mine after my 25 minute ride home from work) so that the Simple Green solution gets help from the steam/heat of the engine to do its thing.