Paul,
There is a core plug hidden behind the bell housing, so you will need to replace it. There is one behind the starter too and I would suspect they all could be corroded at least the lower ones. If you remove the bottom hose and the core plug by starter, plus drain tap, you can use a pressure washer to clean out the sludge, putting it in the core plug hole and flushing forward, it takes a time and a hot wash type will bring better results, wash until water runs clear. Be carefull fitting new plugs as without a special tool you can knock them straight through.
regards, Richard