ive always used a company near me called ribble technologys. they dip ally /steel, iron, basically any metal, into a chemical cleaner. this strips off any oil, grease, carbon, paint, chrome, annodizing, powdercoat. anything at all, and leaves bare metal. best bit is the price, under 30 quid for a complete engine, thats all the casings, head, barrel, crankcase, the lot.
this is a 4 cylinder suzuki engine before the process,
and this is the same engine after cleaning. remember, this only cost just under 30 quid! and turnround was under a week.