Do a compression and a leak down test to determine the state of your motor. Does the it smoke or burn oil? Have you pulled the plugs and checked the condition? Before you start trying to improve the performance you need to have solid fundamentals.
Once that's looked after there are some common upgrades...
292 cam, higher compression pistons, Weber DCOE 45 side draft carbs, Stahl header etc.
For example here's where I'm at. Car currently has stock 1.6 litre motor except for Solex DDH 40 Sidedrafts, and yes those are UNI motorcycle filters which have gotta go (see picture).
The head is currently at the shop getting new valve seals, valves, seats machined, rockers, springs and a 292 cam installed. The motor has major blow by so the block will likely need to be honed and first oversize rings installed. Need to check the condition of the crank and I'm also looking at 1800ti pistons (9.5:1) if I can find 'em. Bottom line is there is no point putting that rebuilt head on a tired bottom end. While the motor is out the car is being sand blasted, rust removed and it's being painted. Should look and perform O.K. next summer but it's a fairly large undertaking.
Check out some of these sites for ideas and pricing. You can also use the search feature on the Roadfly link I sent you and type in key words like "performance upgrades" and you'll get a million ideas from people who know a hell of a lot more than I do.
Other usefull sites include...
General performance parts
Everything Weber here.
The whole Alpina hog can be found here.
The other alternative is you can just leave it alone and enjoy it for a couple of years considering you just bought it. LOL.
Good luck man.
BTW, here's a pic of my motor.