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The classic mechanical governor for ag and industrial equipment is the pierce or hoof governor. Contemporary higher end governors, often of the electronic type are available from woodward, governors of america corp, and certainly others. the links below should get you started, but are in no way exhaustive.
http://www.piercelockcraft.com/
http://www.altfuel.com/governors.htm
http://www.governors-america.com
http://www.foleyengines.com/HandyForms/Long_Range_Universal_Governor.aspx
http://www.surpluscenter.com/item.asp?item=28-1201&catname=engines
http://www.lotus-equip.com/governors.htm
http://www.impcotechnologies.com/
you can also use the gek gcu as a governor. it has timer input for rpm sensing and pwm output for servo valve control