"And I will try to polish it for reduce the white of the plexi that come out after you shaved it with sand paper."
You might start with 1000, then 1500, and finish with 2000 sandpaper. Try to use 'wet or dry' type sandpaper and do the sanding under a small stream of running water to carry the plastic dust and sandpaper grit away.
I usually finish up with Simichrome or Wenol polishing paste on a soft rag. Both are good for polishing small scratches out of crystal tops too.
Milan should have Simichrome and/or Wenol at car care or motorcycle shops.
My first 'real' motorcycle was a black 1964 Ducati 250 Monza...$695 USD brand new.
Ducati 250 GT / 250 Monza (1964-67) technical specifications (moto-data.net)