Opel pistons are not that cheap to do (done it myself and you need N/A c20ex engine pistons for that). They are press fitted to the rods and the block has to be rebored to 86 mm, so in the end you will spend quite a lot for the job. Cheep option is to machine 5 mm of the standard zetec pistons and you will run safe 250 (but that is info for the ZVH build, don't know about whole zetec engine)