I HAVE A 85 CAPRICE NEED HELP ABOUT AC?

HI I HAVE A 85 CAPRICE AND IT HAS NOT HAD AC FOR 3-4 YEARS IT HAS A R 12 AC MOTOR IN BUT IT BLEW ALMOST 4 YEARS AGO AND I WAS GOING TO BUY A R134 AC MOTOR SO I WAS WONDERING IF I HAD TO FLUSH THE AC TO CONVERT IT OR CAN I JUST PUT THE NEW MOTOR IN AND PUT R 134 IN AND LET IT GO OR WHAT DO I NEED TO DO TO MAKE IT WORK? ANY HELP WOULD BE GREAT THANK YOU..SO THE MAIN THING I AM SAASKING IS DO I NEED TO FLUCH MY AC IF ITS BEEN DRYED OUT FOR ABT 4 YRS NOW?