Technically compliant - probably. Legally compliant - probably not. The whole area of compliance is a tower of babble. IoTaWatt has been tested and is compliant in the US (FCC), and Europe (CE). Those certifications are based on Espressif’s FCC and CE testing of the module itself, and subsequent emissions and immunity testing of IoTaWatt.
My opinion is that it would be impractical to consider replacing the ESP8266 in the IoTaWatt. Unlike other devices that use the ESP strictly for WiFi, IoTaWatt uses it for everything.
The ESP8266 is a very capable 80Mhz 32bit processor. I liken using it solely as a WiFi dongle to delivering mail with a Ferrari.
For personal use, If it’s anything like the USA, you can probably just buy one someplace else and start using it. It’s easier to beg forgiveness than to ask permission.
If you are thinking about selling, or commercial installation, you would need to have it tested to the local standards. Cost varies wildly from a few thousand to upwards of $10,000 USD equivalent. It’s not a very transparent industry and very difficult to determine the requirements and cost of compliance testing.