You could do some appliqué to replace something like numbers or letters, however, a tshirt quilt is a memory quilt so why do anything. The shirts show wear and those memories of how they got worn. I have made 25+ tshirt quilts and have never had anyone want me to fix faded designs. I have cut out things like # from the back to add to a front to get more tshirt info on a quilt while keeping the overall size within a range.