Can't open/close window anymore

20755. Caradco. One of these cranks is completely no longer working in one room. Handle came off and is stripped anyway. I had to grab the silver piece inside with a vice grips to turn it (doesn't turn freely) and have someone outside pushing in lightly on the window, just to get it closed before we left the office. I'm extremely handy but can't take the risk of disassembling the landlords window just to try to fix it - would rather have a new opener. Can someone help me get whatever replacement part(s) I need so I can fix it myself? Please email. Thank you!!