Well that could be a lot easier to have made up. Your looking for a flange to flange adapter. Am I correct in assuming that the flange that bolts to the horn has a rectangular hole? The tricky part is making the transition from 1.4" diameter to a rectangle. Unless you have someone with 3D cam software to model the transition and run that portion in a CNC machine it will be costly.
Since the threaded portion unbolts from the horn it may be easier to just use that and have a set of adapters machined on a lathe. They can use your existing adapter as a thread gauge.
Hopefully you have some machinist friends as any of this can get costly.

BillWojo