USB is 5v, you could make or buy a USB3 or USB-C to 12v adaptor as there should be enough current available but that would be separate from the data connection, and if the data is sent at 12v you will kill your PS/2 port if not the motherboard in the computer. Even if 12v works and doesn't break anything you have to hope that the keyboard speaks the standard protocol which it may not as it's very old.
It would probably be cheaper and definitely safer to replace the controller PCB in the keyboard, that's what most people do with old boards.