Any small pump like a grundfos UP15 or a Taco 007 would probably do the trick. Get a model with intergral check valve, else you will need to install one. Since you are on an open system you must use a bronze or stainless steel pump. you cannot use a cast iron.
At 1 GPM, you will exhause a 50 gallon water heater in 35 minutes, and the temperature at the 35 minute point will be down to about 110. Are you sure this system will work?