Frequency Sensitive Switch
If I was tasked with the design requirement to come up with a replacement for the FX301L I would be looking to make a small daughter board that could be retrofitted into the location that the old part was mounted. There are some mixed signal ICs available that include a microcontroller that could be deployed to make the daughter board. 

I would take a look at the Microchip PIC16F570 device which includes an MCU along with a built in dual opamp and a dual comparator. A timer is also available to be used along with software to measure the frequency. The device is available in a 4mm by 4mm UQFN package and can operate with a supply voltage of 2.0 to 5.5 volts. The opamps could be used to amplify the input AC signal to clipped logic levels so that the signal can then be used by the timer logic for the frequency measurement. The part in the UQFN package sells for less than one US$ in quantity 1.