You have a tough problem, but not insolvable.
An appraiser is the way to go. You won't find any comparable cars, which is what insurance companies normally use to set value.
When my sons 951 got hit and Safeco wanted to give him $6,000 for his car - which we figured was worth close to $10,000, we finally got an appraiser out of Oregon - who's name I don't recall. He specialized in classic cars.
We ended up getting close to what it was worth. But holy Christ what a battle!
Here's a summary of our saga:
http://mikepellegrini.com/weblog/2008/08/10/safeco_insurance_sucks/