Video card MC6845 PAL to NTSC

[Zilogoz80]

Hi, I have an 80 column Apple II video card I would like to convert from PAL to NTSC, it is based on the Motorola MC6845, any Idea how can it be changed?, will changing the clock frecuency to the MC6845 be enough or the programing of the internal registers of the MC6845 have to be changed as well?

Any help on this will be apreciated.