crack in coolant reservoir

Epxoy or JB Weld

I would just replace if its old enough to start cracking if you fix the crack its just going to crack again somewhere else.There not that expensive and if you loose your coolant from the bottle cracking you overheat your engine and that could cost you a lot more then a new bottle.

Fit a new container, do not mess about with a second hand unit, you will get the same problem sooner rather than later, also it will be a good time to do a coolant flush as well.