I own some rental property, there are about 30 windows that were installed with aluminum gears in the crank assembly. Pretty much all of them are stripped. They were not designed for long term use. At first I bought replacement cranks but the replacements were aluminum and also broke. My workaround is this instructable.
Step 1: Remove the Crank Assembly
Mine has a removable inner cover, remove the crank handle, these sometime have a set screw near its base. Pop off the crank cover with a screwdriver, look for the screws, unscrew, then look just outside the window and you will see more, remove these too. The crank will need to be extended. The crank arms will have clips that connect to rails on the window, unclip them and the assembly should pull through the opening. When I finish I use the vinyl exterior wood sold at lumberyards to cover the hole. Its easy to cut and waterproof. It matches the white vinyl on my windows too.
Step 2: Buy a Chest Hinge
I use these because the hinge will rotate out of the way when you close it. These are sold at hardware stores.
Step 3: Drill Pilot Holes
Drill two pilot hole for the end of the chest hinge, remove the screw and pivot on the end, you wont need that. Make sure you mount it away from the glass but still concealed when the window is shut. This takes some trial and error. I mount mine just below the latch so its easy to pull then lock the window
Step 4: Make a Hole in the Frame
Mount the new pull then use the end to mark where you need to make a hole. I used a 1/8 drill bit, three holes, then pushed the drill up and down to clean it out. You can use a dremel or more exact bit, but its rental property so I don't get as worried about perfection. It should be pretty tight tolerance to keep out bugs.
Step 5: Woila!
All done. I cleaned the window, put the screen back in, and saved a window! The pull allows you to open and shut without much fuss.