Problem with motor

Im looking to buy this mazda mx3 coupe off of someone but he said theres knocking. He swapped engines with a JDM KLZE. He got the upper head rebuilt last summer and now it has begun to knock he says. He said he has taken it to get looked at again and it seems the past mechanic forgot to tighten a belt and the upper head has warped, says theres rough idle and knocks pretty good. He put 2000$ into the engine replacing all lifters and retainers and seals and its still knocking. I dont know 100% sure whats wrong? Wondering if you knew whats causing the knocking and how much it would cost cdn funds? Thanks