Simple green is the standard, for sure. If you want to speed the drying along, use a leaf blower or an air compressor to get the standing water and the stuff that gets in cracks. Cuts the drying time way down. Do the engine just before a wash and wax, like you would with tires, and wheel wells. That way any dirty over spray is washed away thoroughly. I usually hit the battery terminals with some WD-40 or Marine Lube after cleaning the engine to keep corrosion down too.