you can't cut off the top? I would not want to do this without access to a head tube facing tool. Getting it flat enough to have the headset work right is a very high level of skill. I think I have reasonable hand tool skills, and I suspect the headset would be sub-optimal if I did it. I think the first trick would be a way to mark it. You might be able to use the fork to do that, but I'm not sure how successful that would be.