I can't find an O-ring to replace the one...
I can't find an O-ring to replace the one in the fuel feed valve/quick coupling.
Can you replace the O-ring in the fuel feed valve with a standard O-ring from a hardware or do you have to pay $30 for the extensive repair kit for the sake of one O-ring?
Just so you know, Optimus does make a regular maintenance kit that sells for about $15. That said, if you can match the o-ring exactly, and don't think you'll be needing any of the other, more specific, parts anytime soon, the local hardware store should have what you need for less than a buck.