Don, the or-ring is deeply recessed and hard to see or change from the outside. That is why the starter motor has to come out. Once out I dug my o-ring out with a sharp pick and then pushed the new one in.
I didn't realize there was a starter motor cap so I removed the whole mount, which involved unsoldering some stiff wire 'legs' from a PCB under the cowl cover and made this process a PIA..
Obviously this is not the way to go about it, but I got the job done.
My opinion. Jim