Hmm Paul,
I think you might be going back to Saab's.....as getting a GTA into the USA will be hard.
The A610 was based on development work on the GTA developed for the USA, this was mainly in the area of crash resistance, but CATS etc came as part of the development changes in later cars for Europe- hence the lower power outputs of the D502's/Le Mans cars.
Having had a quick look here:-
http://www.foreignborn.com/visas_imm/en ... ourcar.htm
You can take it there unmodified for 12 months and then take it out again- that is fine.
But getting it into the USA permanently means (from a quick look) :-
Certified emissions upgrades (Lambda/catalyst- Le Mans spec engine at least I would expect, maybe Swiss spec cats too)
Bumper height certification
Anti theft certification ('88 onwards only cars)
The problems is the cost of tests for certification, not the actual work.
BUT there may be a block exemption to some rules that means all cars less then 25 years old, but made before September 1989 may be OK....but this may be just safety requirements, emissions etc are on top of this...see:-
http://www.nhtsa.gov/cars/rules/import/elig082610.pdf
But this document is a bit unclear about RHD vehicles....
Looks like a minefield...good luck, unless you like a challenge and want a unique vehicle when you get there. I know for Smart Roadsters, people have been unsuccessful on trying to work out how to get them in- even though the original Smart is allowed.
Lee