To update the widget yourself, you can copy the mpk file from your widgets folder in your project directory and paste the copy somewhere outside your project directory. Then, you can change the copy's file extension from .mpk to .zip, and you can unzip the widget to access and edit the javascript file in a text editor.
Then, you can copy the updated .js file and return to your widgets folder in your project directory. There you can change the original widget's file extension from .mpk to .zip and click into it to paste your edited .js file (choose "Copy and Replace") to overwrite the original. Then, change the file extension back to .mpk, run your project locally in Studio Pro and Commit your change. Note that the Changes tab will show 0 changes. But when you click to Commit, you can see the file that you modified on the "Changes on disk" tab of the "Commit" pop-up.
You are right that CustomString has been deprecated.
A work around could be to use a DataSource microflow in a DataView, and in the DataView use either the built in Text widgets, or FormatString from the Marketplace to output the text.
Hope this helps.
@Dawn, that will be difficult if the widget has been properly compiled and minified ;-)