Engine cooling system

Hi, I had issue in my car's engine since August 2018. I went to a mechanic, he said it is issue with the engine and head gasket threading. Told me it's better to replace the engine. He put on a new engine but but now the engine heating issue gone, but now the engine temperature is going below the normal temperature while driving and it even take more than thirty minutes to get heated. Mechanic put a new radiator too. Please help me.