first off, you should research how to buy a car from a dealer. anybody with experience will tell you to sell your car first or buy the car and then sell the old one. you NEVER trade in your old car. no matter what. if you can't get the car without the trade in, you can't afford it. you probably paid more than retail or more than you would have from a private seller on craigslist for example.
that being said, i know how it feels to get a raw deal. listen to people with experience here. you aren't going to get them on any type of misrepresentation, so stop trying. the only recourse you have is to get them on selling a car with a swapped engine and not putting that into the paperwork.
i told you before. it's an expensive lesson, and odds are you wont get anything out of this. next time you have the idea to buy a used car from a dealership DEMAND a carfax report. they have the service because they do it before they take a trade in. if they will not GIVE it to you for FREE, walk the fuck out. they are trying to fuck with you.
heres what you do. go in, tell them that is not the original engine that came in the car. tell them that you would like a trade in for a different car or a certified engine replacement. they will not give you a new engine. they will not pay for an engine swap. they will not give you cash. best case scenario you will get back to your original car or a trade in for something else on the lot worth the same amount.