A blast of compressed air into the hole will sometimes free things up.
Some years ago. I adapted a little pool pump to a container and circulated white vinegar through my engine, out the upper rad hose, back into a container, and into the the drain hole. My engine was a non-runner at that point. Probably more practical to use the running water pump to cycle whatever flush solution you prefer.
But before you try flushing, it would be good to remove what you can by prodding around, blasting with air, water, etc.
