For the sake of a base gasket i'd be taking the top end off and seeing what the "real" state of play is. If it's come from the states, crossed the equator and been in a container, surface rust could be an issue. It will also give you a chance to check the bottom end as well. Replacement of the gearbox oil with basic gearbox oil should be fine, but don't be in a rush to start and ride it cause the clutch will be bound up.