I had similar ,classic 36 1999 ,problems this winter ,I took the whole assembly off ,via the nuts securing from underneath .
Take it all home and drilled out heads of Allen bolts ,mine were damaged as well.
I then soaked the assemblies in kettle descale,it can take a week to to release the seized bolts .
I had to replace several plastic parts sheeves and bearings ,they are available online.
If its just the shackle ,remove with a hacksaw.
All back on board now and working smoothly.
Regards Paul Reymond, Nympheas