I love the name "fally downy building", even though it seems to be doing a good job of not (yet) falling down. When my wife wanted to add a roofed deck and future animal space to the back of her house, the then 4 or 5 year old grandkids called it the "barn house"... it is still that.
I think the building is good enough to save, but not enough to spend a lot on upgrading. The beams and rafters are ridiculously badly done; the beams need to be reinforced and possibly straightened, while the rafters can probably work for a long time without modification as long as bounciness is not a major problem.
If you want to straighten the beams for looks or to protect the (replacement) roofing panels, you can make a crude but effective jack with a stiff post just an inch or two longer than the distance from floor to desired height. Put a strong smooth plank on the floor in line with the beam so that you can position the jack post at an angle while touching the center of the beam bottom. Fasten it to the beam so it can't slip but can bend. Hammer the bottom of the brace along the plank to slowly raise the beam to the desired height. Two braces working in opposite directions may make this easier and more secure. Once the brace has straightened the beam, sister on a 2x10 or 2x12, or a pair of them on each side of the beam, depending on how strong it needs to be. You might need to temporarily disconnect the diagonal braces so the beam can flex freely, then reattach them to the new sister beams.
You can use the same method to temporarily raise and support any posts that need to be repaired at the bottom. Lifting the post base just clear of the footing will allow you to clear or replace rotted wood, reinforce the concrete if necessary, insert flashing on top of the concrete, and lower the post back onto the footing. With a small hammer drill you can sink lag bolts into the concrete to tie the posts to the footings. The structure is light enough that this process should not be dangerous.
Once the structure is firmed up, you can determine if the existing roofing can be repaired or if it needs to be replaced.