Code really isn't like a pair of shoes, though.
And I think there's a bit of misunderstanding stemming from the term "engine". It's not like a car engine, where there are specific parts that must be there and where some parts are objectively better than others. It's code. You can modify code to such an extent that it performs so drastically differently that, though it may have been based on code used by the previous "engine", it behaves in very different ways than the previous engine.
It was a comparison. I could have used a car as an example but I wanted it to be funnier. What I'm trying to get at is even if you're changing the code within an engine that ends up making it work differently than usual it is still the same engine. A modified engine but the "same" engine.