Great! Please check the following first:
- iCloud --> any issues opening System Preferences --> iCloud?
- Any problems with FaceTime or Messages?
- Any problems with App Store?
If the serial number in About this Mac is correct, then:
- Mount EFI partition of internal Mojave SSD.
- Backup the existing EFI folder (you can just copy the whole EFI folder to your Documents or Downloads folder).
- Then delete the entire EFI folder from the EFI partition and empty the trash.
- Mount EFI partition of USB flash disk.
- Copy the entire EFI partition from USB flash disk to EFI partition of Mojave SSD.
- Safely eject the USB flash disk.
- Reboot directly from Mojave SSD.
- Does it work?
If it works: MAKE A FULL SYSTEM BACKUP as soon as parts arrive!