What is the ancient attestation to this text and how early is it?
E. F. Hills (see his discussion in KJV Defended, pp. 146-50) says, "almost all the Greek NT manuscripts" include the doxology. He notes it is in codices W (4-5th century) and Sigma and Phi (both 6th century). My copy of the UBS 3rd corrected edition also lists codices K L Theta Pi and family 13 among others.
Hills adds that it is also in an early Christian work called the Apostolic Constitution (4th c.) and is cited by Chrysostom (345-407 AD) and Isidore of Pelusium (370-440 AD).
The key external evidence against it is summed up by Hills: It "is omitted by by Aleph B D S and by six minuscule manuscripts. It is also omitted by all the manuscripts of the Vulgate and by nine manuscripts of the Old Latin. And certain Church Fathers omit it in their expositions of the Lord’s Prayer."
Conclusion: The doxology has credible, ancient attestation in the traditional text of Scripture and should be included in the canonical text of Scripture.