I have a 50 year old house and am replacing the floor tile and have a problem with the original flange for the pipe from the floor and 1 of the bolts is broken do we replace the flange on the pipe or are they 2 pieces/and unscrew the pipe from basement is cast iron

You can buy a flange repair kit at a plumbing supply house or a really good Home Depot. The pipe going into the basement is not going to unscrew. It is probably lead and oakum. Sealed.

Some plumbers like to remove and reinstall cast iron flanges. I like the compression flange with the 3 allen screws on top, bolt or screwing the flange to the floor.