Fastenal has these in grade 12.9 zinc coated. They also have chrome but those are a lot more than the zinc.I wouldn't replace it with the same grade of bolt. Metrics come in
4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9 hardness. 8.8 is likely what it is , which is standard grade most of the time. It's stamped on the head. 10.9 would be a better choice, which incidently is the bolt grade on the later model shock bolts. SAE grade 8 basically. These might have been upgraded in later models too since not a lot of bitching about bent shock bolts lately.