If the owner do not want to change the whole toilet and willing to take the risk(flooding).Looking for replace tank is not easy and is expensive.Then you can try to change the water supply valve to those leak prof valve(when leak happen the water supply will cut off) and fix the crack with epoxy.Or when you leave house turn the water supply valve.

So did the JB Weld work?

I gave my answer before but people said it is not helpful.I don't understand.My way is what ever way to repair the leak.It could work for many years or just brake down tomorrow
.To prevent the serous flooding damage,replace the old water supple valve which is inside the tank with leaking prove valve.This valve is design when the water level down(due to leaking)the floater will stay in place and not come down result the valve shut no water will get in the leaking tank.It will limited the water damage to one tank only.This valve is cheap and easy to replace and can buy in lown.I did this to my crack tank and have peace of mind.If you plan to leave the house for long better shut the water supply valve.Sure the best to change the whole toilet and this is only the option for some conditions(not willing or cannot change)

You-all were responding to a 5 year old thread so I gave you one of your own. While I didn't rate you you got a "reddie" because part of your information was faulty and misleading.
replace tank is not easy and is expensive.This is simply not so. If the tank bolts match up almost any tank can be replaced cheaply. That would be my suggestion. However, Your suggestion to repair the cack with epoxy was bang on.
Then in your last post you fly off on a tangent about the ballcock and "serious flooding". What has that to do with a crack in the tank?
Inquiring minds want to know! Cheers, Tom