published top speed is about 118-120 for the GT model. But most older examples with worn engines....don't expect to get that high.
First of all, you have a 73 spec motor which has slightly lower compression pistons and HP (thank the tree-hugging emission people for that one)
Here's a BRIEF "get-the-opel-up-to-date" minimum you should try if you want more power.
(Go see
www.tgsi.com for a more in depth look)
1: If it has the Solex carb on it toss it out and put on a 32/36 Weber. (consider a 38 DGAS also, if your really bold and port match the intake manifold)
2: Crane XR700 electronic ignition.
3: Increase cylinder compression/ High compression pistons(shave cylinder head)
4: "sportier" camshaft
It's starts to get much more expensive after this....
BTW: headers/exhaust system probably won't make much difference at this point. I wouldn't go there yet....the motor has to be built up a bit more for these to become effective. At a minimum I'd do #1 and #2.