I would start here:
Villiger Stokkebye International
P.O. Box 481938
Charlotte, NC USA 28269
TEL 704.597.0416
Got this contact info from peterstokkebye.com. Since Jorgen Larsen is still with us and the Stokkebye company still exists I'm sure Larsen has the means to make you a replacement stem. I'd bet that you'll have to send the pipe to him and the wait may be a bit, the price will likely not be cheap. If in fact he has completely retired, somebody in the company has the stamp and they should be able to authorize somebody somewhere to produce a replacement. I'd be persistent about that, getting an authorized original replacement. Remind them how much you paid for the pipe......