How is the value of a debt owed by a father to his deceased father (the grandfather) from over 30 years ago assessed, and how is it distributed among the grandfather's heirs, given that some of them have passed away, and knowing that the father left a will to repay the debt?

The debt owed by your father to his father must be repaid. Debt does not expire with the passage of time, no matter how long it takes. If your father left behind assets sufficient to cover his debt, then pay the debt to your grandfather's legal heirs. For any of them who have passed away, pay their share to their own heirs. If the currency your father borrowed is still in circulation, the debt must be repaid in that currency, regardless of any change in its value.