First of all, contact to your previous owner and ask him "Bhai gari meney le li hai ap sey ap ko us k paisey b mil gae hain. ALLAh ap ko wo paisey naseeb karey or mujhey yeh gari. Kindly jo issue koi chal raha tha gari me wo bata do k me us ko thek karwa lo". When did the previous owner changed the engine and transmission in your new car?
In whole this scenario tell us what was the response of the engine, was it jerking? getting high or low revs? car was shutting down? Are you still driving it or its parked somewhere? BTW do you mind sharing for how much did u buy the car ?