Hi Herman, the regulating device is called an AVR and they are commonly available from generator spares suppliers. You could also try ebay. It's worth bearing in mind that if the AVR is failing it's probably due to the fact that it is trying to compensate for a problem with the engine speed so its best to correct that problem first or you will just keep burning out AVRs. Have you tried regular diesel instead of veg oil to see if that improves the rPM? Also, if the combustion is not good most likely your injector will be clogged with carbon deposits. you can clean it with a solvent and wire wool or similar abrasive, then check to see if it is making a proper spray pattern - there should be four jets. wear a breathing mask so you dont inhale the atomized diesel when doing this.
i'm no expert but i suspect veg oil doesnt burn quite as well as diesel even when it's hot. usually bio diesel made from veg oil is processed in various ways to bring its viscosity and flash point to the right specification for diesel engines.