Put a hose pipe in the bottom and flush through with water. Then mix up a medium solution of caustic soda and water, seal up the pipe where the bottom hose goes and fill up the radiator with the solution. Leave for a couple of hours or so and drain out. Flush through with water again. If the old water in the engine was rusty, then replace the radiator and fill up the system with the caustic soda mixture and run the engine until it gets hot. Leave for two hours, and then force clean water through the heater pipes, engine and radiator again until all traces of rust have gone.
Fill the cooling system up with anti-freeze and water and bleed the system.
Steve