Fraud is  the knowing misrepresentation (deliberate lie) of a material (or important fact), made to induce (convince) someone to engage in a transaction (like buying an engine) and which the other person reasonably relied upon in entering into the transaction. If the seller knew or reasonably should have known of the engine's problems, he may have committed fraud and you could sue him to either rescind the transaction (return engine, get your money back) or for  monetary damages (e.g. the  cost of repair). You could potentially sue in small claims court if you think this is the case, where you could act as your own attorney and save on legal fees. You should be able to get sample forms and instructions from or through your small claims court, either in person or online.