From what I gather, the 87 zillas had problems with the rivets loosening on the flywheel. I have seen 2 fixes for this. Replace with a later model flywheel ( over $200 new), or get the existing one welded.
Which is better?
Most people weld them up. I believe the small hubs were replaced by the large hubs which had probloms too.
If the 87 flywheel is welded BEFORE it becomes loose, a welded flywheel will be a reliable flywheel.