I wouldn't swap in a different engine. Get a junkyard engine, pull the head and have new seats installed. Slap it in and call it good.

Not that high an Accumulated milage. Block does not to appear to have much of a top cylinder ridge. Plus lots of what I assume to be original honing still present.

I do think there is an awful amount of erosion damage present as well though.

Blocks don't get a ridge like they used to due to the current high nickel content of cast iron. This failure was a simple fracture of the piston spiroling down hill from there. At the moment the car started to run bad ( one cylinder not firing due to piston top not moving ) is when the piston fractured.

I also see badly burnt oil on the back of piston so I'd expect the rest of the engine to be in poor condition.

If you do get a used engine, consider winding in a set of rod and main bearings as this will increase life dramatically for little cost.