There is no duty. There are various fees to bring the vehicle into compliance but personal effects are duty (and GST) free. If you're unfortunate enough to be moving to Ontario you might get hit with PST though, as Ontario applies PST to used car sales, not sure how it applies to personal effects.
You declare the vehicle on CBSA Form B4 (together with your other personal effects), and you need a compliance letter from the US manufacturer as it explains at length at
www.riv.ca
There is a grace period to have minor changes done, which usually means getting the daytime running lights installed and sometimes having the seat belt anchors re-done. RIV has a deal with Canadian Tire to do this work. Then you can apply for the new title.
If you've got the title it's just a load of bureaucracy you've got to plough through.
Steve.