Sounds like a crystal for a 27MHz unit, which is probably what it will be if it was used for an RC car. To be honest, that will most likely give you very glitchy control over a robot in the arena and you'd be better off buying a new transmitter. They're not as expensive as they used to be; when I started in this hobby I paid £120 for a 40MHz radio system, but now you can get a 2.4GHz transmitter and receiver for £36:

http://www.sussex-model-centre.co.uk/sh ... p?id=29064

The 2.4GHz frequency means that you'll not suffer interference in the arena, leading to good control. You also don't need to fit a protruding aerial in your robot either, which is beneficial if you plan to make a robot that can run both ways up