How do I replace and insert what kind of missing pin - bolt in a home fused disconnect box?
Contact the manufacturer.  These things are not listed for unit repair using random hardware store parts.  Expect Hydro to disapprove any funny looking repairs. 

That is not simply a lever.  There's a mechanism in there, that is important to the ability to "break" high current flows.  

Do not attempt any repair without consulting with the manufacturer. If this is a common thing with an easy and proper fix, they will have it. 

Regardless, this job is a big problem because this equipment is hot at all times.  If it's before the meter, even pulling the meter will not de-energize it; which makes working on it a dicey affair even for an electrician.  This is an unfused load that could pull as much as 25,000 amps straight off the transformer.  At this scale, arc-flash gets serious. 

It is excessively rare for me to say "this should not be DIYed", but if pulling the meter won't de-energize it, I so say.  In that case, to de-energize it, they would need to get in a bucket and go unhook it from the pole line.  Once thus de-energized, have a field day, but again don't do any repairs the manufacturer doesn't endorse. I would sooner just replace the box with one that will be reliable.