Yes, you can define custom notification content within the Settings tab of a PRTG notification. Please navigate to Setup | Account Settings | Notifications and open a desired notification.
You can define custom content in the Send Email(Format) section.
Plain text in combination with PRTG placeholders are valid.
Further information can be found in PRTG's manual.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.