i think if the fitting is cracked, your out of luck. if you have the room and it's on the top, you could cut out the fitting and install an inspection plate in it's place. then get a hole relocation kit for a new discharge hole. if you end up getting a new holding tank, call ronco-plastics.com. they have about 200 different sizes and you can specify where you want all of your holes plus their tanks are pretty thick.
s/v Time Flies