spirts, paint thinner, kerosene or parts cleaner select lowest cost. Clean off with hose and water but be careful at electric connectors so you can wrap these in plastic to protect. As far as finding leaks with engine out cleaning is the first stage. To find leaks you need to pressurize the oil system. So figure a way to pressurize with oil to operation pressure with engine on the stand. Engine builders usually do this before startup on assembled engines by attaching external oil pump to the engine.