A VGA monochrome monitor PCB is going to be easier to hook up to the CRT than modifying the Mac's analog board. That said, it might not be all that hard. The parts that are going to resist bumping the scan rate up to 31KHz from 22 KHz are the flyback transformer and the yoke. It might be necessary to replace them with parts from a mono VGA monitor, and of course all the tuned circuits will need different capacitors, etc.