The valve should come out OK, you may have to use Vice Grips or pliers on the body. You will need to install a 3/4 inch nipple before your valve. I would use a ball valve rather than a gate valve as it would make it easier to flush the water heater.
Maybe you can change out the valve without much water comming out if it is pluged enough in the bottom of the heater.
Once you replace the plastic valve with a Ball Valve and short nipple you will want to take along screwdriver with the ball valve open and poke around inside until the water flushes out the deposits in the bottom of the tank.