Fixing this assembly involves several hours work.
Copy to the Construction Environment and delete the original.
Create a Group for each part.
Change selection filter to Feature Priority.
Move each part to individual group and turn off visibility of group.
Work on each part one-by one to get solid.
or
use to rebuild from scratch (furtunately none of the parts is particularly complicated).
There are a couple of other shortcut tricks depending on what you really need.
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