What are the probable causes to get the Engine overheat?

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I am having Chervolet Venture, model 2004. Recently had failure in engine belt and had replaced with new (alongwith the tensioner). After replacing the engine belt (probably after 1 week), it is observed the engine overheats. Work done to resolve the overheating issue: 1. Cleaned the radiator, 2. Replaced the thermostat, 3. Checked for any hose / pipe leaks. Even after all this excercise, the engine is still getting overheat. Can you please advise the probable cause of engine overheat? Mechanic suspects minor crack in the engine, which is leading to over heat of the engine and he is advice to use HOLTS Wondarweld Heavy Duty repair for cracked engine blocks. Please advice if the above can be used?