The pessimist in me is not sure that roof was stoutly enough framed even before it was modified. It doesn't look like standard pole barn construction, it looks like it was made up on the spot. The pessimist also thinks that when a metal roof sags the nail or screw holes in the sheeting get elongated and it will never not leak. The pessimist also doesn't like what he sees of the wiring.
The optimist in me looks at the framing and sees it's all exposed, you could sister all of the broken, missing and modified pieces from below and get them back to original strength. There's negligible weight on those rafters when there's no snow on the roof, you should be able to jack a sister into place from below.
There really isn't much to a pole barn, they're designed to be cheap, cheap and more cheap. The roof and trusses probably represent about half of the value. How are the poles? I didn't get a feel from the picture. Are they straight? Is there any rot at the base? Any indication of settling? Based on the rest of the building I would be surprised if they were properly engineered, but if they've lasted this long and are in good condition they may be adequate.
I think it's going to be either a cheap job or a teardown. What I would do is a cold-eyed assessment of what kind of building you would end up with if you were able to restore it to its original condition. Are the posts good? Are the rafters and purlins adequately sized? If so, count and measure the number of pieces that need replacing, and price the wood at a lumberyard. Be honest with yourself about how long it would take to insert a sister, and multiply by the number of pieces. Consider the possibility that you might have to reskin the roof and what that would cost.
If you find you have to re-engineer the building it's a lost cause. If you find yourself contemplating new trusses you're going to be happier taking the building down and starting fresh. Depending on how frugal you are you may find it worthwhile to carefully disassemble the building and reuse some of the pieces. The siding and poles are probably sellable on Craigslist, the untreated lumber can be burned.
