R-12 is a pure refrigerant meaning it's not a blend of two or more individual refrigerants which means that you certainly can find Freeze-12, a drop in replacement, for your R-12 mvac. In other words all is not lost if you are not changing your system entirely over to 134-A and simply want to add some refrigerant.
Google search "R-12 drop in replacement" and you'll find a lot of info. Pay attention to what all is recommended be done. Many times a flush is required yet many people have experienced zero consequences without one, simply adding a drop in to the system.