How big is the sash? The bigger they are the harder they are to do.
Measure the hinge where meets the frame. Usually 6", 8" 10" and so on.
The tricky part is working out the thickness of the hinge, Called the stack height either 13mm or 17mm.
As you may not be able to tell the stack for sure and this window is riveted I would suggest buying a pair of each.
You can get friction stay hinges on ebay or
screwfix etc.
Drill out the rivets with a 4.8mm bit and take out the sash. Take lots of pics and place the new hinges along side the old one to judge where your new fixing holes will be (new hinge fixing holes are rarely in the same place as the old holes).
Then you can drill out the rivets on the sash and take off the hinges.
Now is also a good time to offer the new hinges upto the frame and mark any new holes needed there.
Where possible screw rather than rivet so that you can easily make some adjustments.
You really do have to get the new hinge alignment spot on or you could screw up the window.
If you are not confident it may be worth getting a quote from some local double glazing company's.
Good luck.
