The easiest way MIGHT be to install a McHenry. Since the knuckle itself is sprung, the coupler doesn't really need a draft gear. Drill a hole through the shank and pin it in place.
Note that I haven't tried this, as my BN doesn't have many steam locos! But I have seen the same thing done with KD #5s in HO, years ago, and the coupler design is the basically the same.