Water
with 2600-2800 pressure. Blow the grease and crud off. I have used both commercial car-wash and my own pressure washer.
Once clean, I use @ 300-400 grit to make paint adhere. If paint is flaking or non-adhering, I steel brush it all off and primer. On my 4BTA, it suffered from West VA salting, so I cleaned the loose-rust stuff off, painted with POR-15, and gave it Cummins Beige just before dry. Did not POR15 the whole engine, just the rusted areas, non-rusted got only scuffing for paint adhere.
While there are many ways to ultimately get clean, the above method was targeted to an engine that was not to be disassembled.
Wayne