(1) heat the assembly with a propane (or better, if you have one, acetylene) torch to where the shaft is red hot in the area of the screw.
(2) DO NOT QUENCH, allow everything to cool.
(3) Once cool, from the side where the slot was, center, and drill a pilot hole smaller than the inside diameter of the screw thread.
(4) Dull a drill bit slightly larger than the pilot hole.
(5) Using a variable speed drill, just barely start the dull bit.
(6) Now GENTLY jam the dull bit into the brass, and spin the twisted remains of the screw out the back side, preserving the threads in the shaft.

TRY TO AVOID THE TEMPTATION TO USE A NOT-SO-EASY-OUT! wink

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop