89 4runner rear piston cracked (120k)

You will have to check the condition of the cylinder when you remove the damaged piston to see how much damage has been done (if any). If you are lucky, you can essentially just replace the piston and rings. With minor damage, a slight honing may clean up scratches, etc.

If damage is greater, your options may include boring slightly larger and fitting an oversized piston, or sleeving the block, or even discarding the engine in the worst case.

And after knock sensors quit working, they're still caused by severe detonation ;-)