Even if you can find the perpetrators more than likely they are in their 70's and if found guilty would never serve a day in prison until the appeal process ran it course ( long after their deaths ).
I am a firm believer in punishment, but I don't think any good will come of this that justifies exhumation.
Sorry. Very bad logic. Would you apply the same to a 70 year-old who killed a kid today?
And in point of fact, if you are convicted of murder, you go straight to jail, and you stay there while any appeals are being heard.