Then you just need to get your registration card/book be reissued to have new engine number and that's it. Before you change the engine, you need to make sure that engine number is not be already registered with some other vehicle in Excise department's record. And once you change engine you should have new engine number on your registration book/card.
Nothing need to be donefor body parts like doors, seats, dash board etc if cabin and chassis remains the same.
My two cents, I am not expert.