Joel,
As it happens, I was working for a dealer in 1973 (one year before I opened my own shop. Geez, I'm old!)
There is no real MG part number for an MG antenna, as the factory didn't generally supply radios. Antennas in those days were pretty much universal. I imagine you can still get one of these at an auto parts store, though I haven't had occaissopn to do so lately.
The dealers saw radios and the installation thereof, including antennas, as a profit center. If you knew how little the dealer paid for those "stereos" you'd blanch. We could install one, including the antenna, in about twenty minutes. God, how I hated drilling those gaping holes in the lovely fenders of brand new MGs and TRs!
What you're really looking for is an antenna with a base that will roughly match the shape of the surface on which you are going to mount it. Since the dealer was free to mount the antenna any darned place he wanted, I can't make a specific recommendation.
I'm sorry I can't be much more help than that. Anyone else?
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon