My 73 Duster had disc brakes installed in the front by a previous owner….how do I got about figuring out what pads, rotors and calipers fit?

I wanted to put new brakes on the front and I’ve become so accustomed to just putting the year/make into Rockauto and going with the parts they offer and say fit. But since it didn’t originally have front disc brakes how do you all go about figuring this out? Do I take them off and try measuring the rotors? See if there are part numbers on anything?