Not always possible. I don't think you can still get brand new screens for the Bronica system, and most used ones seem to have at least a few marks.

You might want to try the plastic polish or toothpaste on one of the less obtrusive scratches first (say if you have one at the top of the viewfinder), that way if it doesn't help matters you won't make the screen unusable. If it works perfectly then you can attack the rest.