Bronze or copper toilet flanges are still available, and would be the thing you'd attach to the pipe. What is the old one made or? You'd also want to fill in some so that the bolts could anchor to something. To be able to solder it back on would require a few things - buying or borrowing them if you don't have them might make it cheaper to get a plumber to do it. To get a good soldered connection, first you'd have to clean the outside of the pipe and the inside of the flange very well. Then, you'd need a big enough torch so you could get the pipe and flange hot enough to actually melt the solder (this assumes you know how to solder!). The insides of the pipe look fairly clean. They do make a flange that goes on the inside of the pipe (this might be an option if it is a four-inch pipe, don't consider it if it is 3" ID). This has some gaskets that seal to the inside of the pipe. The flange must be anchored to the floor, though, so again, you may need to fill in some around it so it would hold an anchor.