Easiest way to make 'more' power is to fix and tune the existing engine. The carby boxer engine can get pretty sad when it is out of tune, and a specialist with the right equipment is needed to tune it properly. You can only do the other stuff like changing spark plugs, air filter, distributor cap / rotor etc.
If you plan to do anything like an exhaust upgrade or buying a K&N panel filter (yes there's one that suits the twin carb airbox), do that first and then get the carbs tuned to suit. I had an exhaust system done which did not involve replacing the factory headers except for their last 2->1 junction (exhaust shop said they didn't like its design and joining angle). It has one resonator and one sports muffler and it is quiet enough for the street. Never attracts trouble at WOFs etc.
Remove the square plastic pipe which takes cold air into the airbox, separate the two halves of it and throw away the flap etc you find inside. Why? It's faulty and it's not necessary in NZ's climate so it just restricts airflow. Put the pipe back together and seal the holes. There is (or was) a hot air feed from the shroud around the left exhaust headers. Throw all hot air feed stuff away including the rusty, rattly shroud around the headers.
33s are quite basic cars, so it doesn't take too long to take out the interior in order to remove the sound deadening material under the carpet. Sound deadening is a joke in the 33, and this material will probably be soggy thus weighing up to 20kg. While you're doing this, swap the front seats over to give yourself the best condition one, and note you can adjust the seat height / base angle by swapping around or removing the four spacers which sit between the seat rails and the floor.
If you plan to go crazy with mods (you'll probably still have a cheaper car than your mates), have a look at this 33 on TradeMe: Alfa romeo 33ti 1988 for sale - TradeMe.co.nz - New Zealand
You want an engine like that, but hopefully not a boyracer paint job and 17s to slow you down...
The 1.7 version of the boxer engine has larger diameter exhaust headers, carbs and other intake manifold stuff, which you could try making use of once your engine justifies them.
Ooo, and what part of NZ are you in? If North Island, join oldschool.co.nz and I'll see you at the next Taupo track day to improve your driving skills.