Offhand, aside from the janatorial stuff, and associated crap, I don't see that much stuff that dosen't belong there at all..
I see air filters, and air ducts, and assuming that the filters are installed in that room, having a reasonable stock of replacements is not unreasonable.
Supplies for generator maintainance, such as oil, starting fluid, etc are also reasonable, as is a small amount of fuel.
I'm assuming the generator is diesel, with a small day tank, fed by a larger tank downstairs. This arrangement has been known to fail, so having a couple of jerrycans, and a hand pump available is a good idea in case you have to hand carry fuel up from downstairs.
Remember that in a situation where the generator is required to be running, you may not have the option of running out for supplies.
A good preventive maintainance program does wonders, but unless you exercise the generator regularly under full load, you can still have problems.
I would say that an appropriate storage cabinet would be in order, as would proper mounting for the fire extinguishers, but I wouldn't say that everything must go.