I am not familiar with the Tillotson drain plug, but I have repaired a number of Zenith carburetors that had the drain plug stripped.
What I did was salvage a filter plug and unsolder the screen from it. The plug itself has a 5\8" hex, but is shaped like the drain plug with a dished out center, only it is larger.
I drilled out the stripped drain plug and tapped it with a bottoming tap to fit the threads of the filter plug. You have to be careful not to drill into the seat for the main jet.
The result is that instead of a 1\2" hex, you have a 5\8" hex.
Tom Endy