Is a single-engine ferry flight allowed with a twin-engine plane?
Civilian aircraft are not designed for this - but some military aircraft are. The Antonov-70 can easily take off with one engine inoperational (it has four in total), but it has a limited capability to take off with two engines out on the same side! For that, it has to be empty and can carry only minimum fuel.



This capability is the consequence of the demand of the Soviet Air Force for the An-70 to operate for 6 months with minimum maintenance. The Antonov design bureau, well aware of the limited capabilities of front-line troops in aircraft maintenance, designed this transporter with lots of redundancy.



Edit:
With one engine inoperational, any twin-engined civilian aircraft would not be legal if it attempted a one-engine take-off. I would expect there is a way to do it, but it needs the agreement of the owner and some paperwork with the local authorities. If you can prove that this particular ferry flight is safe, I think you would be allowed to fly. But I am certain that without lengthy preparation and paperwork there is no way to legally take off on one engine only.

This is in contrast to military aviation where this kind of operation is planned for.