I know this is a funny question, but I have some vintage RC equipment and I'm wondering if I can interface radio crystals to the arduino. Example of what I mean here. I would prefer not to crack my vehicle open and modify it a lot so that way I can still pick up my transmitter and use it normally also.
Why ?
To control the vehicle autonomously, but have it function normally when I turn off the arduino.
It seems you are confusing microcontrollers with radio transmitters.
You need to do a lot more thinking before you can pose a question here.
The crystal(s) in an RC set determines which radio frequency channel the set uses. Otherwise, the crystal has nothing to do with model control.