I like how all of u are trying to explain this during real-world maths. I really do. but tbh this really doesn't matter. here's a couple points I believe explain why it should be possible to build/restore a ringworld.
1: its a video game, we cant logic everything in it.
2: the game kind of explains this absence of real world science using living metal. this is a super strong self replicating material that actually states "may have had something to do with their advanced construction". this would eliminate the need for a planets worth of materials. here's my thinking.
this is a self replicating material meaning technically u already have an infinite amount of this material. also, if u can use one of this and have enough to upgrade thousands of starships with the material then we must have more than a planets worth of the stuff with just 1 living metal. in the save I'm playing atm I have 14. this should be more than enough to form the base of the ring world. this could then be supplemented with large quantity of minerals. then, using large amounts of energy and minerals could then terraform each section of the ringworld into the 25 gaia. then the ringworld could spawn a lot of living metal as well. and if u think, but isn't that mean I'm mining the ring under me, its self replicating so its growing up through the base of the ringworld.
this is just my thinking, disagree if u feel like it.