Unfortunately Fitbit is aware of this issue with the Charge 2 screen cracking. It does not appear that spare parts are available for replacement [https://topclassactions.com/lawsuit-settlements/consumer-products/health-fitness/882130-fitbit-class-action-says-charge-2-display-screens-prone-cracking/|Here] is an article of someone taking them to court.
I found [https://www.wikihow.com/Remove-Scratches-on-a-FitBit|this method online] to remove scratches with toothpaste. You could also try a [https://www.amazon.com/IQShield-Protector-LiQuidSkin-Coverage-Anti-Bubble/dp/B01LA1AA3O|screen protector] to prevent further scratches. Good luck.