You cannot host your current web service on a non-Biztalk server. This web service is responsible for receiving the message, executing the pipeline and publishing to the message box. All this happens in the IIS Process (the isolated host). You need to have the Biztalk software installed to be able to do this.
You can however write your own web service that runs on another server that will call the Biztalk web service.
Or you could use a reverse proxy server.