I'm with Ian, how did that seat go missing? Usually that seat is pressed in and an interference fit so it shouldn't have come loose. Without the float needle seat there's no mating surface for the float needle to stem the flow of fuel.
That baby ought to be the devil to start with that situation.
That's a Keihin part you might be able to source through a local Honda dealership if you can't find it at Bike Bandit or Sudco.
