There is an ongoing discussion in hpmuseum.org about how to repair scratches in HP calculator plastic screens. I have fixed many, and it is clear for me what is the best solution: to use Polywatch - the plastic version (do not get the glass version - it may destroy your screen).
You apply a drop of this substance on a soft cloth and rub for some time. Remove the white residue with glass cleaner liquid on another tissue - or even a kleenex paper to absorb it. You will see a brilliant screen! This can be used too on you Swatch or Casio watches too.