I now need a gas shut off valve for a Mercury KE3 3.6HP. This is the one just under the gas tank Vince

No one makes that fuel shutoff valve anymore.
The best thing is to rebuild your existing valve.
Sometimes they come up for sale on eBay, but not very often.
The next best is to replace it with a modern style and convert your fuel line from a metal tube to rubber hose. Keep the old parts so you can go back if you find the right type of valve later.


Fuel-Shut-Off-with-Screen

Finding an original gas shut off valve for a Mercury KE3 should not be all that
difficult because the same part number fuel shut off valve assembly is used
on KE3,KF3,KF5,Mark5,KE4,Mark7
Other Green Tank models use a different part number which may/may not fit.
Louis

The shut off valve assembly is attached direct to carburetor.

Are you by chance referring to the fitting with filter that screws into
tank? If so, that would be a toughie as the filter component is often damaged
or missing. In line filter as used on outdoor power equipment can be
installed in fuel line. Those with soldering skills can fabricate a brass
screen filter to go on fitting.
If the fitting filter is clogged soak in lacquer thinner.
Operating without a fuel filter is not recommended.
If fuel line enters compression fitting at an angle other than 90 degrees
the fitting will leak, tightening the fitting to stop a leak may damage
the fitting and not cure the leak.
Louis

I have had pretty good luck making up a new screw.

I roll a piece of copper screen around a nail (used as a form) with some overlap of copper screw.

I then use a large wattage soldering iron to run a bead of solder.

I pinch the end at the top and solder it as well.

I then use JB Weld to insert it into the original brass piece.

I do use an inline filter while running the motor initially until the tank gets clean.

Fine fouling from the tank can still pass through the copper screen so you might have minor periodic problems if you run the motor a lot.