About three years ago I took a 'suitcase' generator as hold baggage. Checking it out beforehand, the biggest stipulation was that there should not be a trace of petrol smell. That achieved, it was accepted without problem.
I've carried outboard parts (more or less a complete 9.9hp minus the powerhead) as checked in luggage in the last year with no problems. wrap it in rags so there are no sharp edges, and then in gladwrap/clingfilm or similar so there are no oil or petrol smells. I didn't drain the gear oil, but it might be worth doing... I've also carried a 20L petrol tank the same way (again, well drained, and left to vent for a few weeks to get rid of any petrol smell, then finally wrapped in plastic).
A used outboard, especially with an integral petrol tank, is classified as dangerous air cargo. This is because it has/has contained petrol and oil. A brand new engine would be fine; a used one would need a degassing certificate before you could ship it by air.