Personally, I still think you should buy a stock one and send them that one, but it's your decision.

I'm not saying they'll cast the piston any larger, but it's safer and more precise with a new one than the in-engine one and to me well worth the extra 60 bucks. I too thought that the 'good' piston would be fine. Oh, and my engine went because of detonation too.
At any rate, good luck and post the results.