You can install a 14.5 OHV briggs on it if you want, however as you get into the 16+ HP engines, the bolt pattern may not match up.  It's not recommended for a mower to use a 16+ engine designed for a 13 HP.  The briggs 12Hp through the 15HP uses the same bolt pattern.  Depending on how bad your motor may be, going a little bit larger may not get the better performance as you may think.  But the choice is yours !!!
Good luck !!
                
 David
                
 July 2007