The use of MIM seems to raise some questions, but long before the use of MIM*, we used to keep at least a dozen or more 870 extractors on hand to replace broken ones. Replacement, as others have said, is easy, but breaking is a nuisance, and they seemed to break routinely.
When replacing, make sure not only that the spring/plunger hole is clean but that there is nothing interfering with the swing of the extractor itself. I also suggest replacing the spring when you replace the extractor.
*I think the originals were made by running a cutter along a steel bar, then slicing the extractors off with a saw.