The short answer is it is unlikely you will get the old and new to blend invisibly. In the olden days there were guys called panel beaters, now they were called panel beaters for a reason. They had the skills and the tools to beat and stretch and straighten sheet metal panels and make them look like new. If you can find one of these mysterious hammer swinging mystics be prepared to sell your first born to pay for the privilege of setting foot in magical cave of creativity. They can be found in the enchanted forest, ask the unicorns for directions.