First thing to do get rid of any oil muck grease with stiff brush or scraper lash on plenty paraffin also protect your coils from getting damp when cleaning.
Also let the paraffin soak into the muck grease for about twenty minutes. I sat my engine in deep roasting tin to clean parts
Don't buy Nitro mors does not work buy a good quality paint stripper then lash it on with paint brush wait till paint blister then a sharp scraper
keep lashing on the paint stripper keep scrapping till its clean like the photo of gear box.
Then wash down with paraffin then clean down with petrol, because the paraffin leaves traces of oil
Then let the petrol dry of outside. I used 1/2" paint brush with long bristle to get undercoat between the fins on the cylinder head.
Photo of engine 1st under coat. after cleaning.
Hope that helps you