I've had good results with S100 Motorcycle cleaner.
Spray on, brush for encrusted crud, hose off. Safe for all types of materials. You can find it at most motorcycle supply places.

One caveat. If you have crud that is covering metal that has lost its protective coating, cleaning the crud away may expose the surface to the air and cause rust.  I've seen this happen in cleaning off old engines that have not been regularly kept clean.

Todd
'88 Westy