Fuel Shutoff Valve screw fell out!

Last week I was backing my 861 Powermaster into the shed and noticed fuel was leaking everywhere. The shutoff screw/bolt/needle (whatchamacallit) had worked it's way out of the fuel shutoff valve, and nowhere to be found!
I thought it'd be an easy fix, but after stopping by Tractor Supply, and surfing the web, I discovered I can't find a replacement anywhere--the closest I got was finding an entire fuel shutoff valve ($35 from Steiners). I have 3 questions:

Where can I find a replacement whatchamacallit that turns the fuel on/off, without having to replace the entire fuel shutoff valve?

Why did it fall out in the first place (is there another component to the fuel shutoff valve that failed, allowing it to vibrate it's way out)?

On a separate post discussing the tendency of fuel to leak from this shutoff switch I noticed someone saying it was the result of a bad O-ring on the end of the bolt (whatchamacallit), and that the entire fuel shutoff valve would have to be removed in order to replace it. Would that be the process in my case too? Will I need to remove the entire valve in order to install the replacement whatchamacallit?

Thanks to those who are willing to help me out with this! My knowledge is lacking in this area, but I'm desperate to get it operational again with snow in the forecast!

Thanks for the help, Tom
Sorry the pics are sideways. Bottom of the fuel tank is on left side of photo (left us up).

I don't know where you could find a shutoff screw without buying the
whole valve.

There's a rivet/pin in the valve that is suppose to stop the screw
from backing out all the way. I'm not sure why yours fell out, maybe
the rivet is missing or didn't stick in far enough.

The reason the valve is usually removed to replace the shutoff screw
is so you can drill out the stop rivet and tap the hole so you can
replace the rivet with a machine screw.

If you search the archives you should find detailed instructions on
replacing the o-ring(s) (can't remember if there's one or two).

Thanks Andy,
That explains why I couldn't find that single component for sale, why it probably fell out in the first place, and why it requires the entire shutoff assembly to be pulled off. Thanks for taking the time to educate me! I'll order one right now.

Do yourself a favor and get a short 7/16 fine thread bolt and screw it in there tight. Then put yourself an inline shutoff down by the carb.
Problem solved plus you don't risk leaking gas on a hot engine.

Sounds like a good tip. Thanks !

The thread size on fuel lines is a special one. It is 7/16-24 UNS; not a standard 7/16-20 UNF one.

Tim Daley(MI)