If you have mechanical skills, it shouldn't be a major deal to replace the valve. I'd go with a 1/4-turn ball valve rather than a globe, gate, or other screw-handled multi-turn type valve. If you turn the pump off then open a faucet somewhere to drain the bladder tank's pressure, you should discharge most of the water in the system. To solder, you need the pipes so that there's no running water, and depending on how the valve is positioned, sometimes a wetvac can help pull out any water that's left. If you have a valve open, any steam produced when trying to solder in a new valve can go out the open valve (the closer, the better, though). If the current valve is screwed on, hopefully, you have a union somewhere nearby, otherwise, you'll need to cut it out, the put new pieces in to fill the gap.