I went to Home Depot and picked up ten 10' sections of 1/2 PVC pipe and a number of 'T' and corner connectors.
I started by removing all of the bent and damaged parts of the greenhouse.
I used zip ties to connect the arches to what was left of the greenhouse structure. The shelves are still usable.
The final step was to create the ridge beam to hold up the center of the roof. I used a 4-way cross in the rear. This allowed me to run a center support down to the ground.
Here's a view of the front. It does not have the center support. It would have made getting in and out too difficult.
Here's what it looked like when finished. I think it'll work.
It was a tight fit to get the plastic back on. I managed to rip a hole in the corner that I fixed temporarily with duct tape (as seen in the picture above). The duct tape didn't work real well and I had to sew the corner with some thread.
When I conceived of this project I imagined all kinds of cross bracing. As I was putting it together I realized that it was going to be unnecessary. Simpler was better. That also means that I have a bunch of PVC pipe and fittings left over. Oh well, I'll probably find something to do with them.