I would think the first thing you would want to do , is to shovel out all the existing ...floor
and " unknown " and wash the whole inside out .
Then go in on a hot day and sit for a while , to see in you were able to get rid of the smell.
I'm just a city boy , but the two worse smelling things that I ever encountered , were a chicken coup and pig barn/pen on a hot day . 80 yrs is a lot of stink
Then and only then , would " I " continue with anything further .
If you get to that point and smell is gone , then I think you have a nice project in front of you .
Looks in the picture like it's sitting on a concrete footing , hence it's stable ( now rereading , I see that you said it's on a footing )
To me , I would figure it's worth time and money to put in a proper floor .
I guess that it's a question of how much it's going to be used .
What kind of expectations do you have for it .
I like the idea of a wood floor better , first , as you stated , it's less expensive, second , you would be able to insulate it better, and third , depending on how you finish the wood floor , it would be in better continuity with the rest of the cabin .
Plus , it also leaves open the possible option of moving/selling it .
Either way , I hope what ever you you works out for you/it . Good luck
ps had a good smile reading the second line in your post .