I based my comments on responses I've seen from rscheitlin who is the master of many/most of the custom widgets that are currently available.
In my opinion only, if he wouldn't attempt it, I wouldn't either. I think basically you would have to rewrite the entire widget and you would have to find all the WAB custom widget dependencies. So, I know you can't plug it in to a JS app....I think you are better off just trying to write it with JS if you aren't using WAB.