I'm not sure he ever provided you a warranty in the first place. It was a used engine and, from the way you describe it, he didn't have or provide a written warranty, he just told you something to the effect of "this used engine may be good for a year or two". If so, that's not a warranty, that's a guess, and you can't hold him "legally responsible" for guessing how long the engine would last.

If he assembled and/or installed  the engine incorrectly, that's another story. Can you prove that he assembled and/or installed  the engine incorrectly? If you simply claim, "He must have assembled and/or installed  the engine incorrectly because it failed" that is not proof of any fault on his part, especially given that it was a used engine in the first place. You would need to provide the court with verifiable proof that the engine failed due to some fault of the mechanic and not due to your misuse, the nature of the engine, or regular usage or wear and tear.