You have two possible options since you said this is sole source and out of scope.  You could use FAR 6.302-1 Only one responsible source and no other supplies or services will satisfy agency needs and get a Justification and Approval (J&A) approved and use a bilateral modification to add the repair to the contract. The second option is a second contract for the repair work, but if it is sole source it will still be awarded to the same contractor. I think the quickest route would be a modification.