It probably won't be that easy. As Lou stated, there were some very small cornet shanks in use in the early part of the 20th century. I have an neat looking old cornet with an extremely small mouthpiece receiver -- modern cornet mouthpieces insert a quarter inch or less. To the OP, I envy that you have a mouthpiece that fits your horn. One option would be to have that shank threaded with Warburton threads. Then you could screw a Kanstul 3C top onto it. Years ago, in a used and antique instrument store, I was told that Dick Akright made reproductions of the old small-shank mouthpieces, but I never pursued it.