although a big gun would be nice
.
Really? what type/calibre? What qualifications are you going to introduce? What tactics/heights are you going to fly?
If the MIB's are in gun range, your in RPG range. And a 1950's airliner is a much bigger target than a tellytubby on a scooter.
Sh1te idea. If I was you, I'd stick to getting the bloody thing airborne to do it's primary role rather than speculative pipe dreaming.