high accuracy crystal for Arduino mega 2560 (clone)

[yeilam]

I need to replace the low accuracy resonator with an high accuracy oscillator.
I need at least 20ppm accuracy.
I don't mind paying for a high quality, preferably crystal, oscillator,
BUT I have no idea how or even where to replace the resonator with it.

Here is an image of the Arduino mega 2560 (clone).
IMHO, The 12MHz crystal is for USB synchronization.
Where is the 16MHz main oscillator hiding?
Anyone has a schematic of the clone board?
Any other help or advise?
Thanks.

[Budvar10]

The resonator is between C5, C6, next to R14.
The schematics is a problem little bit. It is not documented officially, google for some. However, parts around 2560 could be similar to genuine one (schematics here on web).