No repops on a 73 and up. There are people making them for 68 Darts 270/GT/GTS models.
I think you could make one pretty easy. Just need to find out what an original looked like. Did it keep that chrome ring around the control and just filled in. Or was it just one continuous piece in the middle from the surrounding parts.
Just get a piece of textured plastic and cut to fill hole. Blend in with black satin interior paint. Done.