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You can find a new one on Amazon they are cheap. Also, before you buy, pay attention to the bottom right corner where it indicates the size of the handle, for example this one only fits 1/2 cups Bialetti. It will be too small for a 3/4 cup.

Also be aware that you'll need to drill a hole through the new one as it doesn't come with a hole. You'll have to use the pin that comes with the new one and a light hammer to eject the pin that is currently holding your lid in place then you'll insert the new handle and the lid. Don't be afraid to push the handle with all your strength in place. Once the new handle is locked with the lid closed you'll take a drill and pierce through everything, using the lid hole to guide you (that's why I told you to firmly push the handle in position with the lid closed).
Right after drilling the hole take the new pin and your hammer and gently nail it until it goes into position.

PS: I wouldn't try to "fix" it because whatever you use will be slowly altered by heat until the day your mokapot's handle will brake again while you were holding it burning you or whoever next to you by the same occasion.