Road grit picked up from the tire blasts the somewhat dodgy paint off.
KalGard makes some good engine paint.
Oil cooler fittings typically rust, 'normal service' there.
That oil screen cover setup is awful, using button head screws in a high-torque application is nutty.
You can use a sharp cold chisel to make a bit of a groove, and then a blunter one to get the bolt turning.
Remember it's on the bottom, so think twice on directions so you're not tightening it.....
I've not searched specifically for that size, but there is limited availability of high-strength bolts in button head.
I'd search those out before putting the OEM fasteners in.
https://www.mcmaster.com/standard-rounded-head-screwsThere are 10.9 spec bolts there.