If I was you I would find an i parts car and swap parts first before you start tearing your engine apart.
but as for the 88 seta. it has domed pistons like i pistons. the eta pistons are flat. so if you are going to put an i head on your flat top pistons then the combustion chamber will not match which is why i am advising you to stay with your eta pistons and head. now if you want to use the i head do it, it is great even with the eta pistons but for best performance and eficiency use 88 seta pistons.
here is my 2.7i how to thread.
1.3 Motronic Up-Grade & 2.7i How To - R3VLimited Forums