if you REALLY wanted to do it, you'd need to mate the bottom part of a 1.6 sump to the mounting edge of a 1.8 sump, use 1.6 oilpickup, 1.6 flywheel and clutch.

dunno about the best pistons to use but like i mentioned ealier, mike tanski is the man when it comes to big capacity cvhs! he'd be able to advise you!