For clarification:
'A') You have a set of 2.0 P/Cs plus 71mm crank, and you want to install them into your 1.8 ?
OR
'B') You want to put bigger pistons into the 1.8, using existing 66mm crank and rods?
If 'A' you'll also need to new rods due to increase stroke / smaller big end diameter -- the con-rod small end / wrist pins are the same. As long as you use the same 'set' of parts, its all drop in.
If 'B' then get a set of 96mm pistons from Aircooled.net for 66mm stroke and bore your cylinders (much easier!).
Don't forget to re-ignite the jihad-like discussion over use of head gaskets.