I have two things to say on this:
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making a spring is about as easy as it gets. Measure the wire on the spring you have, and buy a length of music wire in the same or next size from a hobby store. Use a piece of tube, rod, or even a wood dowel about 20% smaller than you want the inside diameter of the spring to be, clamp up in a vise and wind away by hand. Give yourself two extra turns for the ends on an extension spring and about 4 extra turns to waste at the ends when hand winding.
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Are you sure you need it? The motor does the least work when the balance (neutral) point is at about half way. The motor does half load to lift to the top and half load to push to the bottom. If the neutral point is at the top, the motor pushes full load to get to the bottom. Ideal is a very long (or effectively so) spring with a low K, so that the change in force is small over the travel to minimize force needed at the ends of travel.