pylon:
You can add an entry in your boards.txt where you change the clock setting to 20MHz, compile again with this board chosen and your clock will even be more accurate than the UNO because you're using a crystal instead of the UNO's ceramic resonator.
Or less accurate. The Arduino Core is specifically designed to work correctly at 1, 2, 4, 8, and 16 MHz clock speeds. At other clock speeds millis and micros are inaccurate. A search of this forum should reveal more details.