What is the best option ...change engine or change gearbox only.....?

i got a HONDA ACCORD 1991 2.0L (MT) ...it works fine but the fuel consumption is a bit high and intent to change to AUTOMATIC TRANSMISSION.....WHAT WOULD BE THE BEST(WISE) OPTION...CHANGE THE ENGINE ALTOGETHER OR ONLY THE GEARBOX...?Some told me if change gearbox only to auto transmission the fuel consumption will be much more higher since the engine(F20A) now using carburetor not fuel injection system....is this true??? ......