For starters, experimentation and more experimentation. Loosen the speeder spring on the external governor linkages. Do a trial run at various speeds and see if it remains stable. There are a number of potential issues: 1.) Engine balance may have to be adjusted to accommodate slower revs. 2.) Governor weights may have to be changed to suit the lower revs if the governor hunts or becomes unstable under load. 3.) Depending on how far you want to change the speed, valve adjustment (to 6/1 specs) may also be necessary to get her properly tuned again.
It should not be too hard to adjust down from 1,000 to 750 rpm. It may be that simply adjusting your external governor spring will be enough. 650rpm will most likely call for a few more adjustments.
dieselgman