I’ve tried to arrange a rap song for piano playing. The problem is that with the rap parts, how can I choose the notes to mimic the rap parts? (I have read this but still want to know what approach to use.)
From the example given by the OP in comments:
If the main accompaniment is gated-reverb percussion and bass, approximate that with unpedaled, nearly staccato, "stride piano" left hand.
If the chorus is sung louder than the verses, in the right hand melody, restrict octave doubling to the chorus.
Fill in other layers as you can, when either hand has a beat free, something like Thalberg's third hand trick.
To really open your ears, study Liszt's piano reductions of the Beethoven symphonies, available as an inexpensive Dover reprint. This wide range of textures easily covers what's found in rap. Study in particular what parts Liszt omitted.