Hi Peter1506,
Thank you for reaching Microsoft Community.
I'm sorry but we do not have a replacement for just the fabric cover of the keyboard available. Please contact support for options available and for further assistance. You may contact support through our Global Customer Service phone numbers (microsoft.com) or through this page: Contact Us - Microsoft Support by using this details below:
1: Open the page then select Get support. Make sure you are signed in with your Microsoft Account.
2: On the Describe your issue here... area, type in Contact Support then click Get Help
3: Scroll down to Need more help section and enter these details on the drop down then click Confirm:
4: After you click Confirm, these options below should appear.
You may select Call me option and you will be ask to select and confirm your phone number then click Confirm. You will then receive a phone call from support in a while.
Selecting Chat with support agent will ask you to confirm the email address associated with your Microsoft Account then click Confirm.