For soap type cleaning, Simply Green, Purple Power, or even dish soap.
I've also used Mineral spirits, and that works well.
For the really baked on stuff I've had good luck with oven cleaning, like Easy Off. Oven Cleaner will discolor aluminum and eat paint.
Some good brushed and elbow grease is always required, I like the longer bristle brushes to get in tight spaces, both plastic and wire brushes.
I have cleaned a few engines and I have not found the ONE method that works best yet.