If it is a deep knock, rebuild it. If it is a higher pitched pinging then reset the valves.
Valves are easy after the first time.
If you are balancing the cost of a rebuild against the cost of a junkyard replacement, they are about the same, but then you are putting in another 20 year old engine... which you will have to rebuild anyway. Better to just get it done before something makes a picture window in the side of the block.

Bill
----------------------------
God Forgives, Rocks Don't...