How can javascript source be altered using a greasemonkey-4 userscript?

Question

The good old Firefox 78 has no support for javascript "private fields". Some modern website javscript code insist on using that feature without a fallback. For example:

        if (o.userAgentDataBuilder) {
          const v = new class NavigatorUAData {
            #p;

            constructor({p, ua}) {
              this.#p = p;

Which generates error:

Uncaught SyntaxError: private fields are not currently supported

Using Greasemonkey I am able to alter html. I have not clue how to alter javascript source. I have also seen questions like Stop execution of Javascript function (client side) or tweak it

How to write a Greasemonkey (4.11) user script for Firefox 78 that is able to alter the javascript "private fields" to non private fields (to use modern javascript in Firefox that lacks private fields? Or is such impossible?