ok.. Lowes & home Depot sell a "torch"(metal) blade from Milwakee
I HAVE HAD THESE BACK-FIRE W/ ME BEFORE THOUGH...
One sure method is a 4.5- 7" angle grinder w/ a steel cutting blade..
(just use caution w/ sparks,etc.)
It will cut (melt) through the pipe quickly & evenly making the repair easier. While the pipe is cut out it would be wise to run a snake through the drain line to remove accumulated sludge (could also have contributed to the crack)..
It is a messy task but will be helpful in adding life to your repaired drain preforming for time to come! Good luck!
CB in Va. Beach