I've seen reports that corrosion in the injector pump return check valve will cause the governor to lower rpm. If you had water in the fuel, It's good bet the check valve is sticking. I don't know your tractor but check if there's an injector return line to the pump as well as a return to the tank. A few pictures of the setup might help. If you can't get access to the check valve, the injector pump will most likely need rebuilt.
Also; if you have a tank shutoff valve, it's more common than not that there's a screen on the fitting that goes into the tank.
I'll see if I can find that.