If you have the missing piece you could try some kind of good epoxy. i've had good luck with jb-weld on the tank of my radiator. The overflow connection is not under pressure, which will help. It does need to hold vacuum for the coolant to get sucked back into the radiator when the system cools off after you park it. If you can find a piece of metal tube that will go in the overflow connection that may work as well. I'd try sealing it in place with RTV, then connect the rubber overflow tube to that.