I removed the left side belt guard on the deck, it had a spring attached to one corner. I am trying to reinstall but I am not sure what the other end of the spring attaches to. My manual does not show it.

It goes to an engagement pulley.

Yea, I don't hink that was a blade guard. I think we frequently refer to that as the spring loaded tensioner pulley. Pull on the end of the spring with a small string through the end loop. Mover it around and eventaully you will see it gets close to but can't reach a worn hole. Remove the belt from the pulley and catch the spring in the hole. Now pull back hard (could be a two man job) on the spring and you should just be able to peel the belt back on. Make sure the balde is disengaged.