Best thing to do is start by covering electrical connections with cling film (some people say foil wrap but not a good idea as it may cause electrical cables to short out)

Then use a good machine/engine degreasant, can use and old paint brush to work it in if needed, then you can jet wash but do it from a good distance away so all you are doing is getting a fine water mist on the engine rather than blasting at it.

For the plastics you can use something like Autoglym vynal and rubber care or a better one would be 303 Aerospace protectant

http://www.polishedbliss.co.uk/acata...nt-engine.html

Hope that helps