When you get to "arpeggio" indicated for the chords, you perform them as arpeggios. How you perform them is up to you because this is the fantasia part of the work. You are given freedom to be creative as long as it's musically suited for the character. "Allegro" just means to play it fast. Note that it is still performed in time because there are measures. If there are no bars, it suggests you can be free in time.