Daniel,
As Michael says practically impossible to get into the US (as you would likely be aware of from bringing in your GT5). Being in Spokane, you may be able to get one imported into Canada (15 yr rule) that is if you were prepared to head across border to drive it and then I guess you could come into the US 'visiting' with it maybe? I'm not sure about registration requirements/restrictions for Non Canadians etc.
Jotec Motorsport in Germany still has the '92 Pantera 90Si advertised for sale at EU98.5K (~US$150K) Jotec 90Si
If you were dead set on a Si Pat Mical has successfully completed at least one all steel conversion and advertises that he sells the panels on his website http://www.patsauto.com There are plenty of donor candidates floating around at present, so maybe a 'project' is in the wind
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