A suitable place for a love song between Fidel and Celia is scene 17b when they decide there is too much at risk to take their relationship to the next level (could start song in scene 42 and reprise later) or could focus on scene 14b and do a song comparing romantic love with brotherly love e.g. a bromance song between Che and Fidel.
You could also write a love song between Fidel and Marita – she was a young girl who hero-worshipped Fidel, but later tried to kill him. See scene 2b and scene 4b and scene 7b and scene 9b to see their storyline – it is Ok to reprise a song.