I have an owner's manual in .pdf format and have no recollection of where I got it. It says the spark plug is Bosch W7D, of which you will likely never find. I did come across a guy a while back who had 51 of them. He no longer has any, but I know who does.
The manual also shows a Champion N9Y plug. I have no idea on the availability of those.
If you want a copy of the manual, shoot me an e-mail address and I will forward.
Don