Doubt it's anything worse than a corroded core plug, especially if the coolant is rusty looking. They are pretty easy to change once you get access to them. I used to tap a sharp screwdriver through the centre of it and lever it out. Make sure the recess in the block is cleaned with a bit of emery and put a smear of sealant around the edge of the new plug making sure it's tapped in square. And not too deep, they usually sit just a fraction below flush with the block.
