Need answer fast.
Well, the first step would be to remove the animal…
I’m kind of wondering if I can just wait it out instead of going into a 2’ high space to wrangle with a maggoty opossum.
I’m thinking leaving it there to rot away is not a good idea. I’m fairly certain it’s going to stink to high heaven right up until it decomposes almost to the point of bare bones, which could be up to a couple of years. Not to mention that entire time you will have maggots, flies, and all sorts of other creepy-crawly critters infesting your walls, looking for something else to munch on.
You’re also going to want to find out where it got in and seal it up, else you may end up with more dead (or living) animals in there.
a opossum will take a while, months and many more if there is low air circulation.
if needed buy 2 8ft. 2"x2" sticks of lumber to reach and grab (like chop sticks) or just crawl all the way in and grab.
the quicker you get it the easier to get near stink wise and also to grab in one piece.
If I were you I’d call an animal control service and see how much they would charge to come remove it for you. It’s worth it to not have to deal with that yourself. Eww.
The first step is to locate it. Then you or somebody has to go after it. The smell will drive you out and bring others in.
Doesn’t Animal welfare or the fire department have some sort of telescopic grabbing stick you can borrow for these type of situations? Or situations where the animal is still alive, that cen be applied here?
or DIY, tie a fishing net to a telescopic window cleaning stick bought at home depot?
Average rat took about two weeks. A possum would take considerably longer given its size, I imagine.
At least it can be accessed, albeit inconveniently. The rat in question was at the bottom of the wall of our shower, under fixtures which couldn’t be moved. Hard to explain. I was just thankfully it was winter and not very humid. We used the other bathroom. 
Another thing to consider is where the decomp fluid is going to end up. I was horrified when it began to leak out from under the wall.
We had a mouse (I assume it was a mouse, anyway) between the wall and the floor, where we could not get at it. There was a tiny space between the joists; we used this space to encase the creature in spray foam insulation. Stopped the smell pretty quickly.
Not sure I’d want to do that with a large critter and a crawlspace though.