Good info here for Americans having a vehicle repaired/upgraded in Canada then returning to the USA.
http://www.cbp.gov/travel/clearing-cbp/automobilesQuoting from the page:
Know that any repairs or accessories acquired abroad for your vehicle must be declared on your return and may be subject to duty.BTW, similar rules for Canadians. Quoting from the
Canadian site:
Under the customs legislation, if you take any item outside of Canada and change it in any way to enhance its condition or value, the CBSA does not consider it to be the same item when you bring it back into the country. You therefore have to declare the full value of the new item.I believe emergency repairs to keep the vehicle mobile are not considered "enhancing its condition or value".