This is just an opinion - I'm no expert!
The skeleton looks quite robust - can't really tell how the newer struts were attached to the old, but it looks as if they're needed. You should be able to get the plywood off - hopefully, they're screwed on, not nailed.
And then what? Glass panes would be quite expensive, heavy and difficult to install. So I'm assuming plastic sheet. You'll need a double layer, with a little airspace between them, so probably secure it with laths and then more laths on the outside for the second layer, because staples will just rip out in the wind.
Could be too big a job, though. Could be you should consider keeping as a nice big workshop or storage space.
