you actually have a few options. The stock Nikki can become a great carb (in the hands of MR. Yaw). however, if you are considering replacing the carb completely for another choice, you have 2 main options.
1. Holley (special modified holley and manifold) can be found used in working condition for ~$250+, new are ~$750)
2. Downdraft Weber (48 or 51 IDA) depending on application, the 51 IDA is way too big for a stock port 12A. and these are pricey.. ~$550 for just the carb plus ~$175-250 for the adapting manifold.
3. Sidedraft Weber (40, 44, 45 DCOE) I actually have a spare one in my garage with about 60 miles on it. Manifold, all brackets and the carb (essentially everything you need to do the swap is ~$800 new).
I have a friend with a 1984 GSL-SE RX-7 we are converting it to a 40 DCOE using a Racing beat adaptor manifold and spare weber parts we found in the garage.
Needless to say, Weber is the way to go, unless you are going to roadrace, in that case have Mr Yaw have your carby