I use (clear) finger nail polish as the Tecumseh Flat Head Service Manual States:
ASSEMBLY
Welch Plugs
To install a new welch plug after cleaning the carburetor,
secure the carburetor in a vise equipped with protective
jaws. Place the welch plug into the receptacle with the
raised portion up. With a punch equal to, or greater than
the size of the plug, merely flatten the plug. Do not dent or
drive the center of the plug below the top surface of the
carburetor. After installation of the welch plug, seal the
outer diameter with finger nail polish or equivalent (diag.
42).
Michael