RE: ParkZone Crystal
Well, receivers come as either AM or FM and will only work with the same type of transmitter. Also, there's the possibility that the pulse width/type may be different from other manufacturers 27 MHz radios.
I'd go along with what Horizon Hobby/ParkZone says just to be on the safe side.
You can always change out the entire radio (tx/rx) system and put in your own. You may have to change out the servos and ESC, too, just like in the Super Cub, but then you'll be able to use different mfgrs equipment.
Just my $0.02.