Last weekend I started the process of replacing the tile floor in my bath. After chiseling out the old
tile I found that the underlayment had gone bad, so I ripped up the underlayment and had to cut it from
around the toilet flange. The flange looks to be soldered to a lead sweep, which is run back to the soil
stack. The flange doesn't have closet bolt slots, the bolts look to be part of the flange, and one of
them has rusted through. The lead pipe comes through the flange and is "folded over" the flange to
form a ring on top of the flange. My questions are :

1) Can the lead sweep be reworked with a new flange, and would it be worth the cost (this doesn;t
look to be a DIY job)?

Yes, the sweep and flange can be replaced with ABS or PVC right into the cast iron pipe. It is not a DIY job. This is a common and difficult job for a plumber but very doable.

2) If a new flange can be worked in, at what point would I have that done. The old flange was installed
on top of the tile, and I'm assuming that the same should be done with the new flange.

Most of the time, the flange is installed on the subflooring, then the underlayment and tile is installed around the flange. If the flange is installed over the tile, make sure that the flange is secured to the subfloor with screws. Drilling the tile can be a bit tricky.

A simple fix but more risky in terms of sealing, would be to replace the existing flange with a solid brass closet floor flange. The lead bend is bent over the flange. I have not seen these in hardware stores. I buy them from plumbing wholesalers. (Not easy to obtain) I try to carry a few in the vans at all time.
Terry