What are some good years to look for, when it comes to looking for a 12a or a 13b?..

 

the newer the engine the better it is likly to be. If a 12A then an 85 might be the best bet. if a 13B a 91 might be your best bet (I Am not including the FD 92-95 because of its noteworthy reliablity problems)

 

'83-85 12As have the lightest rotors.

'81-82 12A have the lightest flywheels (for 12As).

If you're going to go with a 100% stock engine, I'd say go for '81-82, but really any '74-up 12A will be good.

13Bs... really there's the '74-78 (3mm seals, 4 port), the '84-85 GSL-SE (same as '74-78 but different side plates to accomodate 6-port an fuel injection), and after that it gets tricky.. They did a major revision in '86, 2mm seals and different rotor housing shape. After '86 the Turbo engines are 4-port and N/A engines are 6-port. Turbos have lower compression than 6-ports, but there's no reason you can't put a N/A rotating assembly in a Turbo engine if you wanted to. In '89 they upped the compression for both engines. The FD engine is very much like the '89-91 Turbo engine except they altered the intake port geometry and the thrust bearings (to accomodate the FD's pull-type clutch, probably).

There are other differences between '89-up and '86-88 like the OMP type (electronic vs. mechanical) and the FD engines have a different front cover. Plus, each generation of RX-7 mounts the engine in a different place, with 1st-gens mounting the engine by the timing cover like the pre-RX-7 models did.