You can mod it to transmit in 2.4GHz but not what you want in fhss1. They never maked a module for the older transmitter. And even the Chinese never make one for it.
2.4GHz DIY chinese kits that I know still can get easily are Corona and Lemon.
or if you good at soldering, could transplant a low end fhss1 rf board to the 660.
Nothing wrong with using 72MHz, still some new old stock out there, you can get a new synthesized (no crystal needed) 8 ch receiver from China. Or Ebay.
https://www.ebay.com/itm/Corona-RP8D...-127632-2357-0