The thread on the screw should be 6x32. New screws come in the better rebuilding kits.
The old screws may generally be removed by using a diamond burr from the head side to make a centered pilot hole, and then use a very DULL drill bit to "hang" in the screw. When the bit hangs, the screw will then screw out the opposite side.
Floats are also available.
(Opinion) there are other carbs that work better.
Jon.
