I had a similar problem a few months ago. There was a couple of snippets and plugins with hacked code
1) Manager:
- Delete all unused plugins and snippets (like FlexSearch Form) in modx resource manager
- Overwrite with the original code all 3d party snippets and plugin you still uses
2) Ftp:
- delete files/folders of old snippets versions (like old AjaxSearch folder)
- delete and reupload all 3d party snippet files/folders (assets/snippets/snippetfolder) with original files
- upload clean/fresh modx files
or:
(better)
- download your site
- delete all files on remote server
- upload clean/fresh modx files
- upload clean version of 3d party snippets/plugins files
- upload your images, media and templates (after doing a check for extraneous files)
- upload a checked or a clean version of your config.inc
3) Manager:
- Setup/install Modx and ovewrite core snippets/plugins
- "just to be sure" change your admin password again