TO MG Merlin:
Converting my 1962 MGA from Positive to Negative ground was insanely easy. The only part of the car that cared about polarity was the original 1962 radio, which no longer worked (dead tubes for which I could not find replacements). Like all automobile radios, it had a metal case the bolted to the metal behind the dashboard and was expecting to be bolted to a positive ground. As it no longer worked anyhow, I removed it. Then I simply reversed the connections on the car battery. This left me with a red cable attached to the black, negative pole of the battery, and a black cable attached to the red, positive pole. Not a problem as such, the wires are just wires, but liable to badly confuse anyone else working on the car in the future, so I later replaced both cables with correctly colored, black negative, red positive cables. I also added a sticker saying "NEGATIVE GROUND! (just like all modern American cars)"
All of the worries about other electrical componants proved to be false.
Fuel pump, lights, fan, wipers and so on did not care and ran just fine. The generator also worked fine. Reversing the polarity of the battery reversed the direction of current through the field coils of the generator, so the electricity it generated also reversed polarity, that is, came out with negative ground just like the battery. All the arguments we had had about the generator needing to be replaced because it would be double-reversed were just a matter of thinking too hard.
And finally I bought an ordinary 1991 12-volt negative ground AM/FM radio and cassette player, that had the right-sized face plate to fit into the empty space in the dash. Had I wanted greater purity of restoration, I could have mounted the dead original radio and never turned it on, or installed the dash plate that covered the space if you bought the MGA without the radio option back in 1962. But I was willing to compromise a little on the absolute originality of the restoration, to get a reliable radio..
Car and new radio are still running fine.
By the way, if you have an MGB with a positive ground ALTERNATOR not generator (and I am not sure they ever built MGBs with alternators) you may find that it will continue to produce rectified AC with positive ground after you reverse the battery connections. That would be a big problem, and require replacing the alternator.