Given the state of the innards and missing bits of the engine it seems to have suffered a major failure and been donated to us after being ratted for salvageable parts. A fair chance the bung would have gone missing in the process, a few years out in tropical weather has certainly taken a toll.
Testament to the engineering that the reduction gears and front row (sans cylinders) turn fairly freely.
Thank you for the tip on cutting down through the nut, it's certainly worth a shot.