Fixing it should be possible. They do it all the time on Manitous and Al-Megas when their headtube cracks.
The price could be an issue though. Sometimes the frame simply isn't worth the cost of the repair, especially if it has no sentimental value.
Keep in mind that the frame will probably need a partial or full repaint.
You might as well put a collar on it. A good framebuilder should be able to help you out.