Horrible job to do from inside.
Here is an off the wall thought.
Turn it upside down after removing the roof plywood the job then becomes simple. How? hmmm thats the rub. remove the mast lifelines etc get a load of old tyres some old sleepers, a few blokes some straps and an old fashioned dual crane wrecker. That should do it.
You might need to build a false floor to work from though the roof will be weak.