Okay so what you will need is a screw extractor. A proper bike shop should have them and be able to help you with that. Otherwise you should be able to get a set at any decent hardware store, but they should also be used with a power drill.
cheers! worked like a charm! Had to use a saw to fix it, but it worked out in the end.
Here she is. Seatpost needs carbon paste because there is no screw to tighten anything. Pushed some tissues into the seat hole to fill out the gap between seatpost and frame.
Front fork unfortunately still ratlles back and forth, especially when braking. the fork already comes with an angled base, so I could not fit the fork crown race, maybe that's the reason?
If any of you have the same problem or maybe arleady found a solution, it'd be much appreciated if you could share