If the valve is over say 4/5 years old, it might be prudent to change out the whole lot, it will only require a partial drain down, the cost of buying the complete valve may not be much greater than just buying the actuator only, labour costs will be a bit higher but it will be a once off.
You could buy the whole valve and just change the actuator, OK if doing yourself but not cost effective if its not just the actuator.