Short stem is easy -- buy a new one, twist it in with a small box end wrench...It is knurled, NOT threaded, so don't think you'll thread it right in -- it needs some force applied to the end of it as you are twist it in.
Long stem -- take a brake line flaring tool and give the very end a small flare to it (don't get crazy.) Clean the part that will go in the carb body with some scotch-brite or steel wool just to clean it well. Put a very thin coat of red loctite on the outside of the brass stem where the too surfaces will press together. Lightly grab the brass stem with vise grips and tap into place.
If that isn't tight enough, you may have to pin punch very lightly around where the long stem goes into the carb body. I've also used a thin coat of 3M epoxy to help hold it in place.