cultus is a very simply built car, there is no need for any special mechanic to fix it, any good A/C repair works can repair a cultus.
For the radiator, before opening it up for cleaning, flush it out with some citric acid to clean out the internal galleries of the head and block too. Also inspect and replace the coolant intake pipe at the back of the engine if necessary.
If this does not help then remove the radiator and have it cleaned out and sealed back, be sure to neutralize the acid with something caustic like meetha soda - before putting back in the car. The radiator is only part of the cooling system, if the engine galleries are blocked with rust then cleaning the radiator wont do any good, so better flush it first.
Also ensure that the engine always runs at normal operating temperature of 90*C not less and use proper coolant/water mix - this is about half gauge to prevent any such issues in the future.