i do my engine regularly, and my mechanic friend cringes when i do it...hes says you might not have frigged it up this time, but you will for sure. so i too am nervous, but heres what i do.
cover the alternator with a plastic bag. spray some simple green or mean green (dont reach for the Adams APC in the picutre...GARBAGE) on the dirty areas and agitate with various brushes. blow dry with leaf blower and finsih with a coating. here i used sonus trim and motor kote, but other guys ive read love aerospace 303 protectant. i cant say i am not nervous about cleaning the engine bay, but man it looks good when its clean.