It can be fixed, however you are going to have to send it to a custom frame builder who can place it in a jig to keep the alignment correct as the top tube is replaced. There are strength concerns, and depending on the millage, wear concerns as well. It'll take a long time, finding a builder willing to take it on may be a challenge, and I expect that the repair will be as much if not more than the value of the frame.
I think you may find it easier, quicker and likely cheeper to strip the parts, buy a new frame, and hang that one on the wall.