make a new plug for it. Basically trace around the current end of the horn with no plug in it and then use a belt sander to create the tapered oval shape you need. Remember to allow for the thickness of the horn in making the plug. When you are ready to put the plug in, put the end of the horn in warm to hot water for 5 minutes or so to soften it up. I would not use boiling water, just hot water from the tap. Also have the wood grain parallel to the long axis of the horn
cheers mooncoon