In my fiat uno petrol 999cc car on starting the retrofitted AC the engine temp goes above 120 deg

How did you know the temperature of the engine.You can see the radiator fluid temp only and if it goes more than 100 degree then the boiling will start and you car will get stopped automatically.When you start your Ac,then there is a strain introduced to your engine and makes the temp raise.Otherwise your engine is too weak to get the load.A good workshop will do the best.

Cars with OEM AC units are built with larger batteries, alternators, radiators, springs and shocks, etc to handle the increased load. If you just retrofitted an AC unit, the rest of the car can not take the load and increased weight and that is why it is overheating.

retrofitted ac on car put too much strain on the engine and it tends to overheat, and l think the compressor of your AC is more powerful then the required by the 999 cc engine, putting a smaller compressor can solve the problem

try installing a larger radiator or a more powerful radiator fan

Get sum betta cooling

when u charged ur ac its to full & the engine is working to hard also check ur radiator for flow & that both fans r working when the ac is on. next if ur fans donot come on or only one goes on with it hot. u need to check the fan that is not working. by running a hot wire from the battery unplug the fan & touch one end to the fan & a ground to the other it should work it it dose not replace it. if no fans go on when its hot the temp sensor is bad it is on the head next to the upper hose with two wires on it