On my first drill out (hopefully my last for a while), it was also a nice horn worth saving. The applied collar and tip is a good idea. But here is what I did, (as I am not a big fan of the collar).
I still had about 1/2"+ above the start of the inner cavity where it went out the side. To conserve as much length as possible, I filled the hole with epoxy, let it dry, and then starting at the filled hole, I filed down a shaft and cut some male threads for use as a York County screw tip horn. I was able to preserve as much length as possible by incorporating the epoxy-filled area into the threads, which also helped hide the repair. (Looks kind of like a blood spot in the threads, and none of it shows if the tip is screwed on). I think if I ever had to do it again, I would use some horn dust in the epoxy to blend it better. You may be able to do something similar with your horn, if you did not want to go the collar and tip route.
Top Jaw