The utag_data object would need to be referenced before utag.sync.js. If this is the case, you would simply access the data layer elements by typing the following into utag.sync.js
Please note that the utag_data object won't include the automatic data points such as first party cookies because utag.js hasn't run yet. The utag.js library adds these data points once it has loaded.
You would need to move the utag_data code into the head before referencing utag.sync.js. If utag.sync.js is loaded before utag_data, utag.sync.js can't see utag_data.
Keep in mind that you don't need to define the whole utag_data object before referencing utag.sync.js. You only need to include the data points needed for utag.sync.js. The remaining data points can be added to utag_data in the body before referencing the main utag.js library.
Let us know if there are any other questions.