Use a hand-cranked drill with about a 1/4" bit, and gently bore through the crud. The drain petcock hole is a 1/4" pipe thread, so drill bits up to about 3/8" can be inserted without damaging the threads. Just don't get after it with a power drill and bore a hole in a cylinder wall. Once the hole is opened up and flowing, stiff wire can be inserted to dislodge some of the crud.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!