Yeah I didn't know, but should have figured that the border patrol guys ask a bunch of questions like "what are you doing and why."
I just told them I was using the engine for industrial use, specifically a generator on my folks farm...

They asked me what I did for a profession, how much money I had on me, how much I paid for the engine.
I referred to it as an engine, but I am going to say I didn't have to do any paper work on it because I didn't intend to use it in a vehicle.
NAFTA states no taxes on stuff produced and traded between Mexico, USA, and Canada.