I'd assume he could send you a modified new stem as well - but obviously this is far more expensive as you have to buy the stem. Just ask him.
But other than that I am not totally sure about the usefulness of this mod today. It was initially developed by Steve Parry in the 90ies to achieve front suspension (by using a suspended seat post as a stem) and straight bars. At that time only the M stem existed. Today there's S,P,M and H, so pretty much everybody can find a fitting factory stem. I even own such a modded stem, based on an old MK2 stem but did not use it until now.