Please don't get angry at me when I say this but are you sure the odor is from 'dead mice'? Personally, I don't know what a dead mouse smells like after a few days but I can only imagine that a FEW dead ones after a few days in the heat of an attic must be bad! My 'angle', if I may, is can the smell be mold or mildew or something else? If you have mice, chances are you may have OTHER animals (like squirrels or bats or birds to name a few.) Perhaps you smell them.
I guess I'd approach this from a different 'line'. First, you need to plug up any openings. If you have 'large' openings, you have to be careful that whatever may have been getting 'in them' is now outside. Otherwise, you'll have dead squirrels, or something. As you're doing now, look for droppings or nests or 'feathers' or whatever to help determine this. Here's a recent Post on what 'we' offered for getting rid of mice. The same principle applies except you MAY need to deal with 'larger openings'. (I'll leave that up to you.) Keeping Mice Out.
Once your openings are 'plugged' and the dead 'animals' are removed, let the smell air out. YOu can place a small fan in the attic on LOW to help circulate the air. Hopefully, it's vented in some way. Don't use the HIGH setting - It's not necessary. Let it run contiguously for 4 or 5 days. Hopefully, it's dry outside for the most part and sunny. See what happens. If you want to 'clean' up a mess, you can try a bleach and water mixture of 1 cup bleach to 1 gallon of water. DON'T soak anything because you may cause an expansion/contraction problem! Damp-clean it up, and wear gloves, and have ventilation. Let it dry completely before you 'cover' anything over.
As an aside, I don't know of mice leaving behind an odor (other than after they die.) You may find that airing out the house and damp-cleaning up the 'dead areas' is all you need. Error on the side of caution and DON'T seal any walls, or such, until you're certain the odor(s) are gone. Maybe some others have some ideas. My best to ya and hope this helps.
PS: After this afternoon, I won't be back until Tuesday morning but others will be 'around' ...