As far as I know, you can substitute the metal head. You can get the entire assembly for not very much money but you may have to re-use the old mainspring guide seat (the small, flat piece of metal that the guide rod runs through).
http://www.gunpartscorp.com/catalog/...50C&catid=7937
If you'd prefer to stay with a plastic head, you can get either one entire assembly or a package of six.
http://www.gunpartscorp.com/catalog/...70C&catid=7937
http://www.gunpartscorp.com/catalog/...800&catid=7937