My engine has a layer of crud on it from various oil leaks over time.  So much so that the engine smells when it gets hot.  What is the best way to clean it without causing problem by spraying water where it shouldn't go?

ok with the motor cold,  you can spray Superclean on the motor let it sit 5 - 10 min hose it off, and repeat until clean,  
if you are going to drive it right after this - then you want to put a bag of some sort over the Distributer.   superclean will disolve the greese it might take 2  or 3 times if the grees is really thick, or just spray it with super clean let it sit 10 min spray it agin let it sit 10 min more, then hose it off,

I usualy spray the motor down with the super clean let it sit and then start it and hose it with the motor running.

but dont spray COLD water on a HOT block, warm is ok but HOT and it will go stap crackel pop craked block!!

id say make SURE u put a bag over the distrubitor so water dont get in, if it does u gotta take it apart and dry it out

the quarter car wash here has a built in engine degreaser but you have to be careful with your paint job.

Good old Tide will disolve that grease better than any solvent cleaner in a can. Put on an old work glove to protect your hand from sharp edges and work the Tide into that grease. A warm water Tide mixture in a bucket works well also. Hose off and repeat if needed. Invironmentally safe also because Tide breaks down the oil.  

the quarter car wash here has a built in engine degreaser but you have to be careful with your paint job.

The beauty of my Tracker is that I don't have to be careful with my paint job!  A new touch up is just a rattle can away!

Super clean, itl take the paint off the out side of the car if not diluted, it cuts greas / just about anything- including paint if left long enough. really well and the crap at the carwash  in here doesnot work half as good