You can get aggressive with a skinny screwdriver and a hammer into the drain hole to start to break up the clog.
You should be able to get some water to trickle out. That's a good starting point. If you cannot get the water to trickle very much or not at all, you may want to chase the threads with a NPT tap so that you can put the plug back in and drive it until it's time to remove and completely disassembled the engine to do a thorough cleaning of the water jacket.
Who knows how many doses of stop leak Farmer Bob put in there to stop the radiator from leaking over the years.
Whatever they used back then turns into asphalt and is difficult to break up even once you tear it down to a bare engine block.
I haven't found any solvent of any description that will dissolve that stuff.
As long as the engine isn't running hot, I'd continue to drive it as is.
