Refine the Customizer

Customizer is a plugin to PHP CMS Webasyst
The frontend part of the customizer is based on the javasctipt plugin fancyproductdesigner.com (based on fabric.js and implemented as a jQuery plugin). Part of the original functionality is cut by previous developers.
To implement the task, you need to understand Javascript, it is desirable to have experience with Webasyst. It is desirable to organize work with changes through GIT.
A full description of the task in the attached file.