If you modify the extracted files, then delete rename (or delete, I suppose) the original app.asar file, then asar p dest app.asar should recreate the file. The other (more sensible!) option is to simply open the index.html file that is located in the dest/deploy folder (if you extracted to dest) – that will enable you to play the modified game locally, which should be sufficient unless you want Steam achievements to work properly.
In either case, of course, you’ll need to delete the *.json files and optionally recompile the game (using the utilities provided for this purpose with the standard CoG development kit). If present, the .json files are used instead of the .txt files.