I'd take the fender off. You may need these Mercedes SLK R170 Bumper Mudguard Holder Front | eBay
Our you could fabricate as necessary. The linking plates between fender and bumper might also need attention. I fabricated some from carbon fibre tape.
As a first step you need to take the headlamp out anyway so you will get some clues as to whether corrosion is actually an issue.