International flight by Ultralight
It is possible to fly internationally in an ultralight, and many have done so.
Most of the requirements are the same as for a certified aircraft; maps, liferaft, flight plan, sufficient fuel, good nav gear, radio, transponder, passport, passenger manifest, customs declarations, agriculture inspection and customs clearance. (As a rule of thumb, the plane is ready to depart once the weight of the paperwork exceeds the weight of the aircraft.)
But as ultralights are sub-ICAO (below the level that there is international agreement for each country to accept other's registered aircraft), you have to request the other country give you permission to fly in their airspace with your aircraft.
This is not automatic, but they are likely to agree if the type is already flying in their country.