Wondering if the block or head can still be used?
There's no way to tell for sure, but since a true "rod knock" has nothing to do with the block or the head, you could most likely reuse both of them. You'd probably want to do some work to them to ensure they are in good enough shape, but that's another question/answer.
Generally, a "rod knock" occurs when the clearance between the connecting rod journal and the crankshaft rod journal is out of specifications. This can happen for numerous reasons; most common reasons are probably bearing failure, fastener failure, or some sort of physical damage.
If it is truly rod knock, the engine block and heads can probably be reused. However, if the engine has high mileage, it would be foolish to fix the bottom end without rebuilding the heads.
This doesn't mean that the repair will be inexpensive. The chances are that you will have to replace rod, crankshaft, or machine the journals.
If you keep driving and the rod fails, it will eventually have a catastrophic failure; the rod will normally "window" the block (punch a hole through the casting), and/or damaging the valves in the head. In this scenario, the engine is scrap metal.