Chevrolet Optra has been discontinued, however, spare parts are available at authorized Chevrolet service centers. The cost, on the other hand, is comparable to the likes of Honda City and Hyundai Verna.
The Pulley, Clutch and compressor are three different part of the AC unit, and if there is a snag in the pulley, you don't need to change the compressor. Generally, it happens because of the belt or the bearing, but if there is something wrong with the compressor, you will need to get it replaced. We will advise you to visit another workshop to get a better idea on the same.