If its not cooling the first issue is whether the compressor is coming on. If it isn't, the next step is deciding whether it is off because the pressure is too low (low on refrigerant) or whether it is a control problem.
Your car works fairly well converted but it won't work as well as in R-12. I'd leave it alone. If you need a compressor and an evaporator (and hoses, drier, and expansion valve) the bill could get pretty high. We see very few early 126 cars with evap problems. I would definitely get a second opinion. I would advise a high quality A/C shop. Many MB only shops don't have the techniques to save people money. These include manufacturing hoses and leak detecting. One way to be sure of fixing all the leaks is to replace everything. Take it to someone who uses flourescent dye and only repair the obvious leaks first, especially if you convert.
Bosch Master, ASE Master, L1
33 years MB technician