is about as complicated as it gets.
I'd try to find a standard size and modify the window to make it fit, extensively if necessary. A machine shop can maybe help you. Get several bids.
Nineteen of these windows are all over my house and I dread the day I have to deal with this problem.
One answer may be to replace it with a single pane. Nobody may notice and your change in heat loss may be negligible considering all the other sources of heat loss in your house.
IIRC bathroom windows require tempered glass.