Received a damaged engine they refuse to exchange

I recently bought an engine for my Hyundai Sonata from Japan Auto. Aside from the motor being in a terrible state upon receiving it. Once the motor was installed and started for the first time the mechanic noticed a knock in the motor. Once I returned the motor they refused to exchange the motor saying it had overheated. According to the mechanic it's impossible for a knock to develop from overheating.