Do they keep the current engine but replace it with newer parts of they put in a completely brand new engine? Because I was looking at a 20b 3 rotary Mazda RX7 and would like to keep that powerful engine it currently has yet it already has 70,000 miles on it.
The only thing that they reuse is the block. the rest is ever rebuilt or replaced . A good rebuilt engine will last you longer then a new one
The answer to your question is that sometimes they put a bad new engine in the car but most of the time they change the bearings rods and gaskets as well as plane down the heads if there is a warp to them only the person who would know the answer if the engine was replaced with a new one would be the person who owns the car.