it wasn't long ago I was in the same boat. I bought a used 'built' engine knowing it had some issues. Piston looked sikilar to yours. A tear down is definitely rewuired. Once yoj get those pistons out you'll see the damage detonation will do. The top 1/2" of those pistons (side profile) are likely melted. I had a few damaged valves too. tough to tell until you take everything apatt. Hate to be the bearer of bad news but you might be in for some unexpected expenditures. on the positive side ... atleast you didn't put a damaged engine into ur Z.