If the striker plate is made out of aluminum, just have somebody working with aluminum fill in the missing piece by welding. Shouldn´t be that hard to grind it of into the original shape afterwards. Soldering it, if its what you meant and if its at all possible, would probably be too saft material for another round of all of those hits again.
it looks to me like it is made of the famed "pot metal" and not aluminum. if it were aluminum, you could polish a small part of it to a bright aluminum finish. If it is pot metal, it stays gray looking even if polished. I have seen people repair pot metal at hot rod shows with a small torch and a special rod-they made it look easy. I bought a kit and tried to do it myself and failed miserably!