Rich, I did a few things with mine over the years of having it, what you want is to stick with an OE airbox but not the z20let matey you want the 1.9cdti astra box then you can just get a couple of 90 degree bends and fit it straight to the turbo inlet you will still need to keep the MAF just move it's location, so you then remove the huge flat bit over the engine you have now made it shorter run for air to turbo
. Also get a decent panel filter for the 1.9 CDTi box aswell
I believe there is bigger air inlet pipe to the airbox which I think was off a 2.2 vectra that can be used aswell. Better intercooler as you will find the one you have is about 75% covered by the bumper and front cross member so a nice ebay special intercooler and some alloy bends etc..
Bigger tophat is also recomended that the OE jobbie
Decent exhaust and remap at the same time, if you have funds VXR turbo and injectors before the remap should all see you to some good power