I don't think the poppet seals are round profile orings but I suppose it may be possible to stretch a #003 or close over the recess and give it a shot.
I am going to resurrect this thread because I have the same question. I just got a few cornys from an old soda place and need to replace some poppet o-rings.
Also, o-ring size for the pressure release valve would be nice too.
OK, this thread came up on a web search when I posted. Then I see the sticky...so I am not c complete idiot.
Still not a definative answer in that one either.