Simple Green makes something for metals, That or S100 I can hardly think anyone has had their engine in more crap than me riding in winter and all. I always use just a spray cleaner and when dry a shot of Turtle Wax's Liquid polish ICE, I put it in a small (Sample size) pump hair spray bottle and hose down the engine, You can leave it at that or just wipe over it. This ICE polish can go on anything, Metal, painted metal, rubber, plastic, One $14 bottle would last you about 15 years if you used it just for this.
Clean, Taken 9/21/09

You can imagine what it looked like after this.

Clean it, and spray it with ICE it will make it look new, last a LONG time, does not attract dirt AND is unaffected by heat not to mention you can spray it on anything! I also use it on anything Vinyl and polish my tins with it. Especially good on the tank as it does not put white chalk on the rubber gasket around the Dash.