Hi, do some reading first, search this site's archives for posts by Dieseltech and D-239 (engine that is installed in a 574,674,684 and many other models) or Bosch injection pumps.
First quick check, previous owner could have taken pump off without scribing marks and put the pump on 1 revolution out of time or loosened the 3 cap screws on the pump drive hub. If you loosen one of the injector lines at the rear of the pump and there is any fuel dripping when you attempt to start the tractor? If there is fuel dripping and white smoke out the exhaust and engine does not start then there is a possibility the pump is 180 degrees out of time. Solution is to scribe a mark on drive hub and drive gear, remove the 3 cap screws and rotate the hub 1 revolution in the direction of drive gear rotation, align scribe marks and re-install the 3 cap screws and try to start the tractor again.
If there is no fuel to injectors and the STOP/START/RUN control cable is adjusted correctly then there is most likely a more serious problem.
Send me an email at firstname.lastname@example.org and I will email you a couple pages from 684 I&T Shop manual.