It's like the moon rocket. At any time we could have restarted that program from scratch and run it again, but what would the point be? We don't have the direct ability to make one but we have the ability to gain the ability to make one.
If we did regain the capability it would probably be solid-state.
I don't think it's like that at all. I bet you could build a perfectly serviceable replacement at home without needing to spin up any special manufacturing equipment, using off-the-shelf components.
For that matter I'd be somewhat surprised if you can't simply buy a ready-made replacement.
A 500kW LF amplifier? Things do have to be designed to their individual requirements, you can't just buy a car stereo amp and turn it up to 1000.
Maybe something like https://www.nautel.com/products/am-transmitters/nx-series/ ?
Thanks for sharing this, it surely blows the theory about the lost art of building powerful transmitters out of the water.