#16 on the pic above has a very fine pitch thread for its large diameter.
These are known to get rusted inplace but not often. They should have had a light lubing of grease when installed.
I have had luck with one that was frozen inplace on a Tonti frame. Like a bicycle seat post that gets rusted in.
I heated the #16 not the frame, this will cause it to expand crushing corrosion, let it cool, then remove.
This way the finish on the frame is little damaged.
Three chemicals are on my bench.
Nuts-Off... the best penetrating oil ever. Works a treat on salt water outboards. Hard to find & inexpensive.
Rapid-Tap... one of the best all purpose cutting agents. I even use it on files for a real nice finish.
DeOxit... google it, buy it, use it. Put a touch on all electrical connection.
Only in recent years have Italian m/c wiring looms not had issues with corrosion at the wire end & crimp connect. Must be strange alloy.
DeOxit will go inside these connection & do wonders. Keep this in mind.
I doubt the swinger bearing will need changing, just cleaning & repack. New rubber seals might be needed however.
Adjust pre-load on the swinger bearings with the engine mounting bolts torqued and the rear shocks off.
Some pre-load is needed and moving the swinger it can be felt when there is too much.
There might be a spec on this. Like zero + 1/2 turn but IDK.
BTW, i'm 65 soon and have been wrenching since age 10.