Are "This is the Body of Christ" and "I believe" acceptable substitutes for "The Body of Christ" and "Amen"?

A priest has no authority to make creative changes to the Mass; he must follow the General Instruction of the Roman Missal, binding on all Roman Rite celebrations. Canon 846 §1 says the approved liturgical books must be faithfully observed, and no one may add, omit or alter anything on his own authority.

The GIRM specifies:

• Under the species of bread alone, the priest shows the host saying "The Body of Christ," and the communicant answers "Amen" and receives, on the tongue or, where allowed and if he chooses, in the hand, consuming it at once (161).
• By intinction, the priest dips the host in the chalice and says "The Body and Blood of Christ"; the communicant answers "Amen" and receives on the tongue (287).

The USCCB Committee on the Liturgy's Extraordinary Ministers of Holy Communion at Mass states that the only permissible form is "The Body of Christ" for the consecrated bread and "The Blood of Christ" for the consecrated wine, with nothing added and the formula not edited in any way.