Q: Engine has sludge.

Replacing an engine is a very costly repair. Oil sludge often results from contaminated or neglected oil. Not having oil changed at regular specified maintenance intervals will cause sludge to build up inside the engine. Sludge if not excessive can be removed via an oil system flush. A flush consists of a detergent additive that is run through the engine oiling system and these detergents break down the sludge and then the oil system is drained. Unfortunately, though once excessive oil sludge develops in the engine lubricated engine parts become starved of oil and premature wear of components and failure often occurs. Before replacing an engine though I would recommend a second opinion by a mechanic, such as one from YourMechanic, who can determine if a flush can save the engine or if an engine replacement is going to be the only option.