chopin

You may be worried about the plagiarism for other reasons, such as composer's pride, but below is the legality of this situation:

You are legally allowed to copy all of Mozart's music exactly (his original works) and technically as a result, the new score will be called your arrangement and you own copyright. The rule is something like 70 or 80 years after the composer's, or ANY author's death, they lose copyright to their work of art, and it becomes released into the "public domain". That's why you can't copy the Beetle's music, because at least one of the Beetle's is still living today. If you want to own the work's of the Beetle's, the only thing you can do is offer them money like in Michael Jackson's case and buy the copyright. After Michael Jackson's death, if someone else doesn't buy the rights from him, I am pretty sure you would have to wait 70 years until the music becomes free to the public, since the new owner of the Beetle's was MJ.

So there is absolutly no need to be worried legally about plagiarizing from public domain music, because Mozart doesn't own his music anymore. Of course, the only thing you have to be worried about is plagiarizing an arrangement of his music, which may be copyrighted.