What, really, do you mean by the "Mozart of the romantic period"? Because if you are referring the type of music written, that's one thing -- but if you are referring to Mozart's outstanding genius as a composer (of any period) it is a rather different question. In the latter case, in my view neither Chopin nor Mendelssohn, to take two who have been mentioned above, really qualifies (much as I love Chopin...). If you include late Beethoven as being in the period (which can be argued for, quite strongly), the choice is a bit simpler. However, if you have to start later you must also define the end of the period in question with great care; depending on just how you define it, you would need to examine (in no particular order) the likes of Richard Strauss, Mahler, Dvorak, Brahms -- perhaps even Wagner or Verdi -- just to name a few (just a little later you would have to consider some of the Russians, as well as Sibelius and perhaps Nielsen).
So... difficult question!