Excuse this if it's a dumb question.
Are you taking your truck with you from the UK to Florida, buying a new engine and taking everything back OR just buying a new engine in Florida and taking that back with you???
I ask because if you are taking the truck with you then I don't see how you could have any sort of problem returning with the new engine. Just like some have said though, no fuel or oil. I would even go so far as to flush the tank, engine and lines with alcohol first.
If you are just buying an engine then I'd leave the box behind, shove the engine into a zip-loc bag and put it in my luggage and not say a word about it.
In either case I would NOT try to carry it through security, I'd just check it through and be done with it.

Oh, and OS all the way!!!!!!