Even tho civil actions are separate, they are generally based on the outcome of a criminal trial.
Once the government declares 'no harm no foul' in the case of a deceased person, then it would seem reasonable that the civil portion has no merit or evidence of injury.
'I'm not a lawyer, but I've seen one on TV'
I'm not an attorney either, but the usual standard though in criminal cases is "beyond reasonable doubt" while the usual standard in civil cases is "preponderance of the evidence." O.J. Simpson could be not guilty of murder yet be civilly liable for wrongful death.
Anyone who lost money in Enron stock can sue Ken Lay's estate. The only difference this makes (and it is a significant difference) is that the plaintiffs will have to prove in the civil case that Lay committed securities fraud; had his conviction not been vacated by his death, his guilt would have already been proven and would be treated as already established in the civil case.
There is some talk that the plaintiffs' lawyers may not pursue Lay's estate, but that has more to do with the perception that the estate may be insolvent than with a legal inability to pursue the case.