The part number you need is GA Spares PTC807

They are the same size, and often marketed, as replacement o rings for Victa brass fuel taps through ODK. Maybe give Bruce a call

https://www.outdoorking-forum.com.au/engines/victa-engines-parts/carburetor-fuel-parts/victa-fuel-tap-o-ring

Would also suggest one of the gaskets as you can always guarantee it will split
https://www.outdoorking-forum.com.au/engines/victa-engines-parts/gaskets/victa-g3-carby-bowl-cro3269a

Regards
Tyler