The short term solution is an ozone generator, usually under $100. Close the place up, let the generator run for an hour or two, let it sit for another hour, and then open it up and let it air for an hour before you go in.
In the long term, it probably needs ventilation.
I have a $60 I bought on Amazon and does wonders for musty and mildew odors in my workshop. Its lasts maybe 2 or 3 weeks, then I run it again.
I saw a video where a guy had one of those, put it on a wasp nest that was in the ground, put an aquarium over the machine and the hole and ran if for about an hour. Killed almost every wasp.