You have to trade off either the baud rate or your 'precision' timing to the servo. If you must use 19200 baud then you really require a 11.0592 mhz crystal or else your communications won't work - which would make the whole item useless! I can't see why it would be difficult to alter your timing parameters using the 11.0592mhz crystal apart from the fact that each time tick is not an integer 1us. I doubt if the servo requires absolute timing precision - they're not guaranteed to be precise anyhow! A microsecond here or there for the servo will not cause any perceptable error so you can round up your timing calculations.