I am not up to speed on any modern stuff, especially MTH offerings. I have to presume your tender is a bare-bones tender that is empty inside. It looks like what you want to attempt would be more work & money that it's worth. I am strictly Loionel, and you seem to want a Lionel style whistle, which operates through a relay and roller pickups on the trucks. There are different styles of whistle from Postwar era, that could work. You would need enough room inside the tender to mount it, whether it is vertical style, or horizontal. Depending on style, you need to cut a hole for the sound to escape. Another hole would be needed for the pickup wire from roller on the truck up to whistle. If this is the direction you are attempting, it seems more trouble and parts than it would be worth.