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.
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.
@Dawn, that will be difficult if the widget has been properly compiled and minified ;-)