You're never going to be able to offset that angle without physically modifying the frame. An intergrated headset is out of the question as your headtube would have to be machined internally to act as the cup as bearing fits directly inside the head tube. A zero stack headset could help but would only offset the top tube by a few mm. You could also jam a larger tire in the rear to bump the angle up a little bit but that's also a change of a few mm. If it bothers you that much, a 650c fork with accompanying wheel would set the top tube straight, but also lower your bottom bracket height and increasing pedal strike.