Depends on the radio the airplane comes with... it could be as simple as buying the new radio on the same frequency and off you go. it is also possible that the receiver int he airplane is compatible, but the wiring order of the reciever be different so you may have to swap which servo s plugged to which channel. It couls also be that the receiver has the wrong "shift" and you need to replace the reciever.

If its REALLY junky... then the servos may be hard wired to the reciever, which would mean replacing all of the radio system to use the new transmitter.