At this point, the engine has to come out and be replaced.
You'll have to decide if you want to spend the time (and or $$) to have the engine disassembled to find the cause.
When I had my last engine failure, I simply bought a used/salvage long block, had all of the ancillary components swapped over, and had it re-installed. $4K out the door including the long block. You just have to find a shop that is willing to do the work for a reasonable rate rather than gouge customers to pay off their summer vacation homes.
