use a grinder and a wafer style cut off wheel to cut the cast. understand that the cast is going to be heavier than it looks. use dwv-drain waste and vent pvc to replace make sure you hang the pipe every 4' and use the proper size fernco couplings to tie the cast and pvc together. there is nothing worse than hearing a cast line crush under the load of a pair of soil pipe snappers. If the pipe was good it probably would not be leaking and then i would say rent a pair of snappers. read the lettering on the pipe you are buying somewhere it is going to say dwv or sch 40 go or even cell core you can use any one but the pipe will be white in color.