So it is not the whole control system you want to replace, just the power failure/recovery detection. I guess it is possible to replace this with a mains failure relay as a simplest approach. Ideally it will detect over-voltage, under-voltage, and no voltage, but with appropriate delays to reduce nuisance starts.
The link below looks like something basic. Its output is a changeover contact (form C). A simple circuit could be a contactor with an appropriate 120VAC or 240VAC coil, or a small DC power supply (12 or 24V) that operates a relay with the same DC coil voltage, as long as power is present. It probably needs mains rated contacts. This relay contact will connect to the circuit wherever the old 3 phase mains failure detection connected. The advantage of this small power supply unit is that you can later add battery back-up for a fail buzzer, time delays, threshold settings etc if you wish.
There may need to be a similar modification to the part of the auto start system that detects the generator output is valid.