Assuming that it is registered to both owners, all you can do at this point is buy the half that belongs to the living co-owner. You could then probably operate the airplane without interference for a while until the dead guy's estate gets organized.

But if he passed away recently, his estate will likley be in probate for a while, so I doubt you could quickly close a deal on his half. I'm not sure I'd want to be co-owner with an estate. You never know what the estate might try to do...ground the airplane or freeze all spending until they figure out what they want to do. You might be on the hook for all Mx and annual expenses if the estate does not authorize those.