How to install flange on irregular drain pipe

[tugweltp]

Hey all, (super novice here) how can I install a flange on this old (cast iron, lead?) toilet sewer pipe (1962 home) when the interior of the pipe hole is irregular and not a perfect circle to fit flanges on the market? If you look at the pictures, the sewer drain has some sort of growth, deposit, or buildup making the pipe ovoid and irregular. Is this calcium buildup that I can chip away without damaging the old pipe, or is this something else like perhaps metal or concrete that a previous plumber poured? If I can't chip the irregularity away without damaging the pipe, is there like a flexible flange that I c can shove down in there? Thanks!