You do not need a crystal unless you want the frequency to be very accurate. If you do need it, the frequency of the crystal has to be 16MHz or less. 33MHz will not work.
You cannot use ADC to generate PWM. You may use the Timer to do it. See TI code samples.
http://www-s.ti.com/sc/techzip/SLAC081.zip (for assembly code) or
http://www-s.ti.com/sc/techzip/SLAC080.zip (for C code)
Read the “Readme.txt” file first to find what the code sample files are for. Look for file names with"ta" (TimerA)