Shouldn't be any problem with pressure washing motor - keep the high pressure water well away from electricals though (which fortunately your motor doesn't have much of.
The thing you really have to watch is which degreaser you use to clean up with. A lot of them are amazingly caustic, which not only eats away at your hands, but puts a white surface corrosion on the aluminium parts of your motor.
The best one I have found is one recommended by Mercedes, made by Autoglym and it's called "engine and machine cleaner". This stuff is brilliant! Not only does it avoid the caustic problem, but I don't think I've ever used a product that cleans up an engine bay this well.
Hope that's some help - and no I don't work for Autoglym 
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