I have just put up a video today on Youtube showing the swinging arm bolt with the head cut off. With the help of Harry and Gaetano, in the end. Harry used a disc cutter to fit in between the frame and the swinging arm. This was made by pushing the frame slightly out. How you get yours out is up to you. Looking on the web I have heard of people using force, waiting for the WD40 to penetrate the rusted bolt.
If you look at a parts pdf, you will see that either side of the roller bearings and needle bearings on the swinging arm. Ducat fit a metal shim either side in the shape of a bobbin. Such as the ones on the back of a swinging arm to lift the bike up. As these rust and corrode over time. They are extremely hard to remove.
In the end using a disc cutter and pushing the frame out enough to get a cutting disc. The swinging arm bolt was cut.
I hope you read this before you service your bike. Or have it to do before it gets seized. That way if ever you carry out any work on the swinging arm. Or if you check your suspension etc. It is caught early.
Now Mick from Metropolis Motorcycles, (Ask Mick about his paintings if you call him) supplied me all the following parts new.
77210021AB screw. 7840241AB washer. 77159382B nut. 36910031A screw. 77210031A fork pin. 93050121A swinging arm bolt. 70140171A. ROLLERS CAGE. 70250451A BEARING. 71010231A BUSHING. 71010241A BUSHING. 71614141A SPACER. 71210191A SPACER. 71510031A SPACER. 93040111A OIL SEAL. 70140041A BEARING NEEDLE. 71310781A SPACER. 70250451A BEARING. 85210681B SPACER. I also replaced the bearings on the suspension top which was also worn. Check for spares your self and hope the video helps.
The bike is going to handle much better than before. i hope this helps you think about servicing before it needs it. Or to carry this out as a service task to prevent any difficulties in the future.