This is the first instance I've heard of where the push rod tubes have rusted through .
But then I live in a desert environment, and rust is not a common occurrence here !!
I would say though, that if the push rod tubes rusted through, the rest of the engine may not be in much better condition .
I'm probably going to pull it apart and get all the castings vapour blasted. Mildly tempted to look for the later heads (or possibly a motor) as I believe the valves are a bit bigger. But, it fires up and sounds ok.
Apart from the steady dripping noise as oil runs out of the pushrod tubes....