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.