Aye, AFAIK more or less everything is different on the 1.6 when compared with a 1.8 (regardless of turbo or not).
I've had an 8L 1.6 and my good lady has hers (only just back on the road after suffering the effects of chocolate rivets in the gearbox) - and they're a fair enough little motor, but very different from the rest. As above, loads of the suspension/running gear is different - hubs and most of the braking gubbins differ (the caliper carriers are integrated on the 1.6 as I found out when a joey garage mangled the thread for one of my caliper bolts with a windy gun - thank God for Helicoil!), gearbox/drivetrain too of course. It's a fair shout above that it'd be cheaper to just get a 1.8T 'from-the-factory' model. To be honest, with the prices they're going for these days you could probably hang on to the 1.6 for sentimental reasons, buy a 1.8T and still be up on the deal when compared to retrofitting the engine.
Good luck getting your head gasket sorted! Let us know how you get on...
Regards,
Rob.