Slider door lock

It's very difficult to determine what mortise you need without the original on hand to use as a reference. Your particular handle is compatible with about a million different mortise styles, so there are too many to narrow it down at this stage.

The best place to start would probably be with the size of the opening. Please tell me the exact width and height of the oval opening, as well as the distance between the two smaller screw holes at the top and bottom. When measuring those screw holes, please make sure to only measure from the center of each hole, not their edges.