For feature parity, the old PC will require USB OTG, HDMI input, wiring for ATX control, and a software stack.

Sipeed makes a PCIe KVM card for around $80 that drops into standard PC cases.

I'd assume it runs off the 5v standby power when the primary ATX supply is sleeping. =3

https://github.com/sipeed/NanoKVM?tab=readme-ov-file