Most likely the steering tube is pressed in, and so it can also be pressed out and subsequently replaced by and machine shop or, as suggested, frame builder.
Sometimes improvising a little bit is the most cost effective way to proceed. The purpose of the top nut is to preload the bearings of the headset, providing the radial and axial stability the application requires. If you can find an appropriately sized spacer and engage the thread where it's not stripped, go for it . You might even put a dab of loctite on those threads to keep it from backing out ever.