You could simply buy some 6mm PTFE rod (I think it's 6mm), trim level, (with very sharp knife) to length and replace the existing white (pretty sure it's PTFE). Very cheap and you will have enough rod to do lots of valve stems. You should be able to easily remove the existing seating.
http://stores.ebay.co.uk/PTFE-UK-...ubZ315924018QQtZkmif that doesn't take your fancy, I think this is probably the same part:
http://www.nuovaricambi.it/caffem...SPRESSO%20MACHINESPart Number 560069
and the two o rings part number 616686
If you don't fancy doing either, as the valves are so cheap, why not simply replace it?
P.S. If you want to keep it really cheap, you might be able to get the existing one out and turn it around, if the back is not in good condition, I suppose you could pack it out a bit and re trim it level
