The details regarding shipping of parts from Canada into the US is probably not best answered in the Canadian forum. I have looked into this and generally all should be simple enough (and duty-free), but if it is an engine, then potentially the US EPA gets involved.
As for importing parts into Canada, if the parts are specifically for a car that is 25 or more years old, then those parts are Duty-Free. Tax and brokerage fees (as dependant on method of delivery) are applicable. Typically the Canadian taxes on import would be either the GST at 5%, or HST at around 13%, but this is dependant on destination as different rates apply to different provinces.
I hope this helps. Feel free to PM me if you have more specifics.