For the outside of the engine, making sure all openings are covered or plugged to prevent dirt from getting inside the block, degreaser, scrapers, and nylon or brass wire brushes work well.
For the inside of the block, unless it is interfering with the normal operation of the engine like restricting oil drain back to the pan, personally I don't like to mess with it because whatever you loosen up can later get caught in the oil pump and oil passages and then into bearing surfaces.