This a standard petcock found on round bowl carbs.
First step is to drill out the rivet with a 3/32 drill bit.
Mark the bit to prevent drilling to deep. Don't forget to
remember the orintation of the bezel plate.
Then use a 5/16 drill to finish removing the rivet head. Be
carefull not to drill to deep on the bezel.
Then pry the bezel off of the body.
This is why they don't completely shut off the fuel flow.
You can hand hold the body to tap it as it is soft metal
and easy to tap. Don't forget to lube the tap. The tap size
is 3mm x .50. If you don't have metric use a standard
4-40 tap. The drill bit is the same for both. 3/32.
Use a small file to round off the sides of the rivets to make
reinsralling the bezel over them.
Reinstall the bezel and insert the screws.
That's it, a $5 fix instead of buying a new one.