As far as I know there are no "off the shelf" fit pistons suitable for any 1.8 zetec (OEM pistons that is).

For a 2ltr block (bored to 86mm - aka "2.1"), you have the choice of from Vauxhall:

Calibra Turbo (LET) Pistons, suitable for 16v Zetec turbos (CR too low for 16v!)

Astra GTE XE pistons, suitable for ZVH 8v conversions (CR too high for 16v!)

2ltr blocks arnt much these days!