the Formula atlantic 4ag is based on the 16 valve 4ag as seen the first iD series. these engines rev to 11,000+ these aren't generally BUILT by trd but are required to use TRD-USA controlled cams and crankshaft. these engines make about 240hp in race-legal tune.
-these are supposed to be mounted in an open wheel race car in the formula atlantic champoinship and are probably not streetable like in initial D.
the GroupA 20valve as seen in second stage was intended for use in a steel-bodied production based race car so it's alot more Streetable and should be driveable on the street like in the show. These engines DO come from trd but anyone sufficiently rich could build a similar unit which also makes 255hp. Contrary to popular belief there is no reason why a well maintained GroupA wouldn't survive as a daily driver.