Hello,
The
A600 schematics on page 4 (video) have an extra circuit for PAL users so that the Composite/RF output is the correct frequency. Not too sure what would happen if you disabled it.
X1, the system clock should be changed from 28.37512 MHz to 28.63636 MHz.
U12, the CXA1145 video encoder has an NTSC/PAL pin. I think removing R203 and fitting in the space of R202, will force NTSC. On the A1200 which has NTSC/PAL selectable, this pin is controlled by the A1200s _NTSC signal.
In answer to your original question, there is no easy way! :-D
