Page 16, post #227 of this thread refers:
Spare hammer spring, bay item 231352543722, 0.9 mm x 70 mm.
I ordered the one in stainless.
Unlike the stock spring, the above replacement has pointed/bare ends. I flattened the ends down carefully (using pliers) to prevent the ends gouging into the hammer and end cap. There are probably better ways of doing it.