The statute of limitations simply means the offending person cannot be prosecuted for the crime itself. I have no love for the Fed, but I do have love for our nation of laws, and if the property was stolen, it should be returned to its rightful owner, Fed or not; eighty years ago or not.
That being said - because of the collector value of the coins, the Fed stands to benefit more from selling the coins then melting them down and the surviving family should be offered some compensation for the find.
RE: Feds Snatch $80 Million in Rare Gold Coins from a Family Inheritance