How to repair a corrupted Sketch file destroyed by a program crash on Catalina?

I am using Sketch on Catalina. After a crash of Sketch my document (412mb) could not be opened any more.

I need this document to be opened to save having to redo a week's work. Is there any way to repair a corrupted Sketch file - like adding any binary block to the end of the file, deleteing some cache-part in the file or elsewhere etc.? Or maybe by using SketchTools?

Edit

Using Sketchtool (the first time) I get this output:

./sketchtool list layers /path/to/file.sketch

2019-11-20 18:44:33.410 sketchtool[3427:485317] DB Error: 26 "file is not a database"
2019-11-20 18:44:33.410 sketchtool[3427:485317] DB Query: PRAGMA journal_mode=OFF
2019-11-20 18:44:33.410 sketchtool[3427:485317] DB Path:  /path/to/file.sketch
Failed to open document file:///path/to/file.sketch

(SketchTool:1)

{
  "pages" : [

  ]
}

Unzipping the .sketch file works.

OK, I restored the file by unzipping it, comparing it to an earlier version (I luckily made a backup file) and copying the missing file document.json before re-zipping it to a .sketch format. After opening it Sketch corrected what was necessary and I could use the content again.

To be on the safe side, I copied the latest version of the work into another sketch file.