Lead bends were (and sometimes still are) fairly common way to go to a toilet flange.
Do you have access from below? If so, I suggest you cut out back to where the lead bend connects and replace the whole thing with new. Once the lead bend is split, I think you'll always have problems. The lead bend probably connects into a cast iron hub. It's my understanding that they typically leaded in a brass stub, then attached the lead bend to it. You can remove the entire bit and substitute pvc. They make a rubber donut that will fit inside that hub and pvc will fit inside of the donut hole. If you get the right size donut, it works fine. The alternative is to have a plumber replace the lead bend, or do the whole thing for you in pvc or abs.
When you're done, ideally, the new flange is installed on top of the finished floor and anchored through it into the subflooring. Assuming you go pvc, you want a flange with a ss ring so it won't rust out and use either brass or ss screws to anchor it.