The M113A2 was fielded in the late '70s to early '80s. Some improvements included a new engine and tranny with new locations of the fan and radiator to improve cooling, higher-strength torsion bars and a relocated rear idler wheel (raised) to increase ground clearance, and shock absorbers to reduce the effects of ground strikes. With the new engine, a new internal rear engine access panel was added that was in two pieces as opposed to the one-piece A1 panel. Threaded holes and footman loops were added along the hull sides to allow attaching ERA panels as well.
Tamiya has a new"ish" M113A2 kit (35265), which is not totally accurate since they just added sprues to their earlier M113 and M113A1 kits. There is no interior in the kit. The biggest issue is the rear idler wheel is too low on the hull. On the A2, it was raised for better travel of the track. Further, the box top shows an M113A3 and one of the options and parts are provided for an M113A3 with the external fuel tanks (only on US A3s), but doesn't really have any other M113A3 parts. So it is confused at best.
Academy's M113A2 (1354) kit is pretty good and has a pretty accurate A2 interior as well. It also includes figures, weapons, and gear to fill it up. It also comes with the external fuel tanks as well. The rear idler wheel is is still too low on it though.
Academy has also done an M113A3 (13211) that is pretty good. It too comes with a full interior with the addition of spall liners on the inside as an A3 has. It still has the laterals of earlier models though. A3s have steering yokes like an aircraft and other modern armored vehicles. It is pretty accurate, but still needs some work to be correct. The rear idler is still too low on it too.
Academy also does a very nice Vietnam M113A1 (1389). It has the correct parts(inside and out) for an A1. The low idler wheel is corret on this one. It also comes with a full interior which has the correct part for an A1 as well (one piece rere engine cover door). It also comes with crewmen, a couple infantrymen, a Vietnamese woman, extra weapons, and gear.
I prefer the Academy ones. They all also contain both vinyl and indi-link plastic tracks. I would use the M577-specific parts from the Tamiya kit and add them to the Academy kit, using the interior from it as well. You will end up with a much better model. Then you can build the Tamiya one as a straight M113A1 using the left over academy parts.
Good luck.