First swap the PAL VIC-II chip for the NTSC version (MOS 6569 for a 6567). The VIC-II chip is the one with the heatsink compound on the id plate. Click Image for detail view.

Next replace the PAL 17.73447 KDS crystal at Y1 for the 14.31818 KDS NTSC equivalent

Last, change the jumper from LK4 to LK5.

The Commodore P500 pictured here still uses 220V (English) current and the UART is still European, but the video has been successfully converted to NTSC. If the the picture needs adjustment carefully turn the potentiometer near U21 to fine tune the video. There is another pot nearby, not sure what it does.