Since you don't know what type of gas it use, put a electronic thermometer in the evaporator coil. Set the low-pressure control to cut-In at 38*f evaporator and cut out for whatever you need to obtain the temp you want (15 - 20*f coil temp is typical for this type of cooler usually, to obtain 33-35*f box)
Or just tell the customer, they need a new temperature control to make it get colder. Then install a Ranco A12-700 in the coil. Problem solved.