The other guides are fine, but you can get this without spending money and having to be the required rep level to do an engine swap.
After you complete chapter 2 the next campaign mission Lucas will ask you to go try to convince someone to sell you a part. When you select this mission you will be tasked with winning a race with a vehicle that he has left for you at that location. The achievement popped for me once I won that race as it apparently had a swapped out engine.
I completed this at rep 18 and for the car I have I need rep 23 to swap my engine.