Just found something halfway useful. If the pistons are OEM, they should have the word TOYOTA cast into them in the side, near the wristpin hole. I'll attach some pics.
Also, looking at the
TRD Japan website, I don't see any 81.5mm pistons, and the "0.50" mark in the piston crown almost certainly indicates an overbore to 81.5mm (81.0mm is stock).
How exactly is the car's existing GZE "blown"? What's wrong with it? In other words, do you really need the NA's pistons, block, etc?