One of my relatives place (old 3rm of more than 30 year old), the window is the louvre kind with metal plates at the bottom and top in glass. It is a corridor unit and has 4 windows, two each in the living and bedroom. The hinges holding the metal and glass flaps are not in sinc. Some flaps are loose and cannot be locked.

Is there any company that will change just the parts required?

Looking at the lowest possible cost to repair it. Can DIY? Or how much does it cost to get it replaced?

quite diffcult. but even you get one, they dun come cheap i guess

The frame is cold-dip galvanize finishing steel, it rust and together the accumulation of dirt jammed the hinge.

I have not try, maybe you brush off the dirt trapped on the hinge, loosen the rust with WD40 and lubricate it.