In the background you provided you stated the repairs are out of scope repairs.  Even if you had the Changes Clause in your contract you cannot use it for out of scope changes.  Out of scope repairs can be handled in one of two possible ways.  The first and probably the best is a new contract that is competed and has a well written performance work statement that covers all of the identified out of scope repairs from the first contract. The second possible way of addressing this problem can only be done if you have an approved Justification and Approval and the contractor agrees to a bilateral modification to the original contract adding what was the out of scope repairs.