It’s important to mark the kill lever so it indexes back to it’s original position. I marked the center of the stud and the lever as well.
The arm will pop off because the spring is acting on it. So be careful.
Remove the spring and the shim above the oring.
Then dig out the oring carefully. Install a new oring and then reassemble everything.
Push the new oring into it’s recess with a socket.
Here is the orign fully seated.
Add the washer back on top of the oring.