Stan Wilder wrote:

REMEMBER NAFTA?

You will have duty charges only on the improvements made to the engine. I
don't have a Hormonized Tariff Schedule right in front of me but I
suggest that you call US Customs office near you. They'll mail you the
forms or tell you where you can download them.

Never attempt to bypass Customs Authorities, even simple mistakes cause
delays. There are heavy fines, penalties and possible prosecution for
mis-representing your import or export.