if it was done right, there should be R134 quick release type connections installed over the old R12 types. That and a note attached specifying the oil used and the amount of R134 used.
Assuming all that was not done, the next option would be to completely evacuate the existing gases, drain the oil from the compressor, flush the entire system, prob get a new condenser, a universal fit parallel flow would be good, new drier, new hoses, clean the filter at the joint of the hard lines of the high side near the firewall (there is a small screen in there), evac the system and check for leaks, then recharge with about 80% IIRC the amount recommended for R12.
If you are unlucky, then the expansion valve could be bad, generally speaking you can tell this by odd reading of the gauges assuming everything else is ok.
IF the expansion valve is bad, then you have to decide if you want to risk an used one if you can find it, OR pop for a new evap unit that uses a standard easy to get expansion valve., that will set you back about 250-350 if you can find one (they are out there new).
Cheap route would be just to evacuate, flush, drain oil, new drier, new oil (ester), pull vacuum, charge and hope.