It is relatively possible to import your car - here's the details.
US Octane rating is R+M/2 - not all countries follow that - what does method does Australia use?
Shipping should cost you (including crate and insurance) about USD$5,000.
Customs will probably hold your vehicle until you have US EPA and DOT approval to remove it (EPA is Environmental Protection Agency and DOT is Department of Transportation). Obtaining this is complicated, and requires the assistance of a DOT certified automotive importer. To find one, search for "DOT importer" in
http://www.google.com .
The tests you have to pass to drive your car on public roads are simple: emissions and crash.
In your case, since the S13 - S15 were sold in the United States, you do not have to crash test your car. You do, however, have to have a DOT-certified importers statement that your vehicle has been modified to meet DOT crash test standards (the strictest in the world, by the way) for the same year/make/model of vehicle sold in the US.
Emissions is the same - since the S13-S15 chassis were sold over here, you only have to have a DOT-certified importer state that your vehicle passes emissions for the same year/make/model of vehicle that was sold in the US.
The SR20DET will NOT pass emissions in the United States.
In all honesty, I know this because I looked at importing a '99 JDM FD3S (RX-7) into the US. I did not do so because the cost is prohibitive.
Your best bet is to crate up your car, ship it to the US in parts, purchase a USDM 240SX of same year, and have a local tuner shop assist you in installing the SR20DET into your USDM 240SX along with assorted accessories.
I cannot speak for the cost of making the car pass Colorado-specific emissions, as I live in Texas. Remember, each state has their own emissions laws, even broken down to the county level (here in Texas, various counties have different requirements).
Unless you have lots of money to spend, strip your car of parts, ship it to the US, and install them in a USDM 240. If you have a lot of spare cash, spend it on importing a more rare car - 95+ FD, Skyline, etc.
Questions welcomed at
[email protected] in case you have further inquiries.