I'm almost certain you can't make a PAL amiga into an NTSC amiga or vice versa without replacing the RF-Composite circuitry. They are different systems, besides having slightly different clocks and the 50/60Hz issue (or non issue, because as mentioned, monitors will display 50/60Hz anyway).
It looks like you are correct paul1981
According to
http://www.wonkity.com/~wblock/a1200/a1200.html"Q. Is there an internal jumper on the A1200 to set it to default to PAL or NTSC video?
A. No, but if you're really motivated, you can rig this up. My thanks to Tetsuo Oda and Byron Montgomerie, who provided the information that connecting pin 41 of the Alice custom chip to ground causes the A1200 to default to NTSC, while pulling this pin high (disconnecting it from the motherboard and connecting it to +5V through a 4.7k resistor) will make the default power-up state PAL. I would point out that making a modification like this on a surface-mount chip is difficult at best, and you may end up needing an expensive motherboard replacement, or, at worst, a dead A1200 that needs an expensive replacement motherboard available. In most cases, using the boot menu (obtained by pressing both mouse buttons on power-up or reset) to switch to PAL is sufficient. However, with some games, the hardware patch will be necessary to insure proper timing (changing to PAL via the boot menu might make a 50 Hz game run at 60 Hz timing, or vice versa). "
Sorry, for the miss info, the A2000, A3000, A4000 all have regular NTSC/PAL jumpers
The A500 has a easy to install jumper wire in the later models, but it looks like the A1200 is not so easy
The A1200 is the only common amiga I don't have.