If the engine is in the car and it still is running you could use 1 Qt. of ATF poured into the oil. ATF has alot of the cleaners that use to be put in motor oil to prevent sludge buildup in the first place. I know guys that still add a Qt. everytime they change oil and swear by it. You pull their valve covers and it spotless.
Just remember anything that gets cleaned off drops into the oil pan. Anything in the oil pan gets pumped thru the bearings (unless you've blocked off the oil filter bypass). Sometimes things are better left alone until done right.