i think the fact that it was a former US resident car is very significant. heres a passage from US Homeland Security:
Re-Importing A Previously Exported Vehicle
A vehicle taken from the United States for non-commercial, private use may be returned duty free by proving to U.S. Customs that it was previously owned and registered in the United States. This proof may be a state-issued registration card for the automobile or a bill of sale for the car from a U.S. dealer. Repairs or accessories acquired abroad for your vehicle must be declared on your return and may be subject to duty.
Importing a Motor Vehicle - CBP.gov
the Homeland Security page seems to be fairly comprehensive (safety, emissions, customs, etc).
if the car was pre-1988, the ontario owner might have removed/altered the emissions equipment. but you dont have that worry if the car is 1990s, cause they're still subject to regular emissions tests (every 2 years).
edit...oops...my comments were directed at Targa911S post, rather than Jrboulder