Purge valve nipple snapped?

If only the end section has snapped and there is still some of the stem left, get some brass tube with the same outside diameter plus some the next size up.

A piece of the larger size can be used to make a joining sleeve that fits over the remaining part, then an appropriate piece of the smaller one inside that to make the new connecting stub tube.

Glue the parts together or solder the brass parts first then glue over the plastic.

Example tubing:

https://www.ebay.co.uk/itm/Brass-tube-Hollow-CZ121...

If the tube part has broken off completely, you may still be able to make a part but you would have to make a flange that joined to the valve body.

A few brass washers soldered around the tube at one end may work to give a disc that could be drilled & screwed, or clamped with screws around the outside.

You have to buy the whole purge valve assembly.  Get one from a junkyard.

sounds painful ...