I'd consider a new O ring mandatory every time you remove the pump. They're so cheap it's not worth risking a leak. Lock ring is optional. Old one can be re-used. For the fuel fittings the cheap Chinese tools work great. You can make a tool out of a beer can or piece of tin flashing to do the job in a pinch.

 

o ring should come with the pump. Lock rings usually get destroyed upon removal in the rust belt.

 

I must be exceptionally lucky then. I've re-used three.

I've also never bought a tank that didn't also come with the O-ring.

 

You are lucky. Then again I wack the tabs on it with a chisel to get them off.