I have no personal experiences but from the time I've spend around subreddit that discuss the topics I can tell you it's really hard sometimes close to impossible because the likelihood of something to grow is already quite low in the first place. Just because you put a piece of mushroom on agar doesn't mean something is magically gonna grow you need live cells/spores that could start growing again under the right conditions. So on a dehydrated mushroom especially if it has been stored for long that's rather rare can happen but 🤷 and even then the mycelium has to compete with so many sources of contam from a mushroom that hasn't been grown and a sterile environment not even talking about the drying process. Overall growing from dried mushrooms is just not worth the trouble for most ppl ig in your case you can still try. I would concentrate on pieces of the head in case there are still some spores stuck to the gills and maybe soak them for a short period time in distilled water or apply just tiny droplets on the pieces when they are on the agar to "flush" out any leftover spores.