In my younger years i used to use diesel, a paint brush and hot water.
Dip your paint brush un a tub of diesel, brush off the dirt from your engine bay and then rinse it down with hot water. (Cold water does work but not as good) The diesel acts as a de-greaser and the water washes it all away.
It takes about 1 hour but it looks great when its done. Costs buttons (£1.00 of diesel)
You can buy engine laquer so when its all done and dry then spray the laquer over the engine and it dries like a shiney dick
cover up up any electrical boxes.
Cover up air filter and air intakes.
Dont put too much water by spart plugs.
Your engine may well misfire once your finished but dont worry as its just the damp and the damp will evaporate. It may be worth taking it for a good run to warm the engine as the damp will evaporate faster with heat.