[Saga40]

Yes there are 2 unpopulated locations on the board. After doing some research, it looks like one of the missing chips is the mag.

[Mateusz]

You can buy HMC5883L chip alone, or such a board http://www.ebay.com/itm/GY-273-HMC5883L-Triple-Axis-Compass-Magnetometer-Sensor-Module-For-Arduino-3V-5V-/400362466832
Just remember that either you orient it the same way as it is on Revo, but if you orient it differently then you need to dial in degrees of rotation with respect to Revo's magnetometer in GCS.
Then you just check on scopes if two magnetometers match.

This link I provided is just first I found,but there might be cheaper boards, I don't think it should cost more than $2 including shipping. You would need to supply voltage preferably from GPS to power that mag.