Overfill the crankcase with about 2 or 3 quarts of kerosene or Diesel fuel added to the oil, and run the engine at a fast idle (1500 RPM or so) for about 30 minutes. DO NOT DRIVE or put any load on the engine while the diluted oil is onboard! The crankshaft and rods splashing into the over full crankcase will scrub any sludge and rust from the bottom end and the piston rings more effectively than you could ever do with a brush. Drain the oil while it's hot, change the filter, and refill with good quality 15W-40 oil. You won't do any damage, and it just might free up the rings.
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!