I'm having one of our machinists at work do mine. He machined the throttle body to 50.8 mm and this was ages ago. he worked out the angle of the butterfly to make for it but hasnt done it yet.
If you strip yours down, you should be able to get a machine shop to make it for you. Once it's bored out, put the BRV elbow on, scribe or mark the new bore on and port it to match along with the inlet manifold.