Is the 12a a two rotor? What about the 13b?
Is a pretty big deal to go from a 12a to a 13b, or a newer rotary?

 

The 12a is a two rotor, so is the 13b. The two engines are almost the same except that the 13b has slightly wider rotors, and therefore more displacement. In production nearly all 13b's were fuel injected and nearly all 12a's were carbureted.

It's common to swap 12a's and 13b's, the process isn't hard, but it does take some time and there are some tricks to it.

 

The numbers represent the approximate displacement of the engine. 12A is approx. 1.2 litres, (though the actual displacement should be rounded to 1.1) and the 13B is 1.3 litres.

I was told that the A and B represent the fuel system type. I was told A is carbuerated and B is fuel injection. My brother told me this back in '96 but I thought I once read that the letter stood for something else, though I can't remember what.

Sometimes you will see 13BT or 12AT. The T stands for turbo. 12A engines were made turbo from the factory, though they weren't shipped to the US.

 

12A > 1146cc > 1.1L
13B > 1308cc > 1.3L

as for the lettering, i think it's just a progression from A to B.