Oh thank you for the photo, that certainly clarifies the thread direction. A picture is indeed worth an abundance of words.
Cutting the nut off is definitely now part of the equation, though on removing the crankshaft we have discovered that all the rear timing gear and cams missing plus missing chunks of gear teeth.
Our idea of animating the engine will also require some further thought as our original plan of modifying the starter gear to run off an external electric motor has been scuppered by the absence of a lot of vital connections at the rear. All good fun.
Once again thank you all for your input.