I called the airline in advance and they told me that only unused engines could fly. Once you put gasoline in an engine, you can't take them on a flight, not even down with your luggage, because in the confined air inside a plane, passengers could smell the gas odor and complain.
So, a week later I got to the counter to check in with two motors. One brand new 4 stroke in it's box unused and a used two stroke that I cleaned off of all gas and oil residue in the carb and engine; and packed it in a box too. I dissambled the carb on the two stroke and cleaned it dry with acetone as good as posible using paper towels, hoping to get the gasoline smell out of there and packed it in a zipbag that I labeled with a part # so it seemed to be an unused replacement part. I found couple Misubishi stickers and stuck them to the exhaust and intake ports so it looked unused and also kept the smell in.
To my surprise the used two stroke engine was allowed to fly with me as luggage. The four stroke they did NOT allow to fly because there was some oil left in the crankcase; and to fly, no liquids are allowed in the engine. I tried to explain them that the oil inside the crankcase was unlikely to drip or escape, but after 9/11 the rules are what they are. With one hour for my flight to depart I had to rush on a taxi to a friend's house near LAX and ask him to UPS it for me. I barely made it back to catch my flight.
Take your brand new engine out of the box and drain ALL liquids, factory oil included, or send it UPS.