Thanks for all the input.
I contacted everyone that was suggested:
R&E, Bilenky, Clockwork Bikes, Tandems East,...
They could sell me a "tandem" fork for $300-400, but nobody could give me any specs on what made it stronger than a standard.
I contacted Dimension (the one my LBS ordered) and haven't heard back.
I contacted Scot Nichols at Ibis (the original designer of the bike) and he couldn't remember anything about it and didn't have any records.
My LBS did the repair and found that the 1" threaded fork had a piece brazed on to it (maybe for extra strength?). It was a kluge. He had to get creative to build it back with today's available parts.
I posted pictures of the damaged fork, the rebuild, and some old shots of the bike in action in this photo album (I'm a newbie to bike forums and don't have permission to submit links yet, so add the front)
photos.app.goo.gl/ndHUWcZMav9KJxkx8
This is my first time using Bike Forums, it's been very helpful.
Next, I'm looking for a full suspension MTB tandem.
Please provide any input (I've already contacted Ventana and Alex Nutt at MTB Tandems).
Feel free to point me to a thread or give me guidance on starting a new thread.