Using my A code 22" Green MBH to illustrate this very simple how-to.
This is how I rescued this kettle many years ago. First, drill out rivets.
Be careful not to enlarge the holes. Start with smaller drill bit and increase in size till the rivet pops/falls out.
Here are my replacement vents I had. You can buy replacement vents or use donor kettle vents. If you require fasteners, stick to 1/2" long 1/4-20 round head ss bolts, nuts, and washers. Btw, every nuts and bolts on any Weber kettles are 1/4-20.
Here it is all finished.
Same procedure for replacing damaged lid vent. I would use carriage ss bolt instead of round head with slot or star for cleaner look. Need to take off some material off the square part of the carriage bolt but worth it IMO.