Although this is a forum to assist folks doing DIY plumbing jobs, sometimes the best advice is to call a plumber. I say this partly because it appears you are working with an older installation, and working with CI is not an easy DIY job. A plumber would make short work of this including installing a new flange. Of course, there is a minimum service charge even on a very quick and easy job, but often we DIYer get in over our heads and it ends up costing more in wasted time, parts, and often having to call for help anyway. For example, if you "whack it with a hammer and chisel" and the darn pipe breaks, then what will you do?