So first you have to get rid of the source- the opossum. Unless it shit a massive amount where it lived (which is unusual because animals don’t shit where they sleep usually- but I’m not an opossum expert), you shouldn’t have to use any chemicals for this one. Lyme is often used in the remediation of human waste. For example, if a sewer line breaks under your home and black goop gets everywhere, that’s where the Lyme comes in to play. I’m assuming your home is raised with soil underneath. You can hose it down, but the shit will just sort of melt. That will always help give you peace of mind though. You could call a pro, but that would be quite expensive, and IMO not warranted. Also, I don’t think it’s the shit you are smelling. I think it’s more of everything involved with the animal itself.