Just to throw out the obvious: it should be trivial to get some old Node code up and running, particularly if you don't mind using an old runtime version. If you've run npm shrinkwrap you have a log of the exact versions of each package used. You don't have to use ES6 or Typescript, you can keep on trucking the same as you ever did.

It sounds like the author was an ASP.NET developer, dabbled a little in Node, then worked with Xamarin (also .NET) then moved to ASP.NET Core. Nothing wrong with any of that, but it only stands to reason you'll find ASP.NET easier to work with than Node when you've used it extensively and Node not very much at all.