The fuel button is located inside the governor spring cover directly at the rear of the injection pump. There is a square cover on the back, it should still have safety wire on the bolts, and you will see a pipe plug on the hump of the cover. You need to remove the 4 bolts and carefully remove the cover, be cautious not to ruin the gasket, there will be fuel that runs out when you do this.
You will see a plunger under this cover, this is the high speed governor and idle speed control. Carefully remove the snap ring directly at the rear of the governor, Keep one finger on the spring pack so you do not drop any of the parts!!!! There will be a stack of shims that controls spring pressure for the rpm governor, remove them and set aside along with the snap ring and heavy washer and then main spring.
You will have a clear sight of a set screw in the back of the plunger, do not turn it just pull it strait out the rear. You will now have the plunger assembly in your hand which houses the fuel button right in the front, switch it with the new on and reinstall in reverse order.
The most important part is to not drop shims. I would have liked to have some photos of doing mine but I had fuel on my hands and was not about to dirty the camera!
About a 10 minute project as long as you don't drop parts, or drop the snap ring and have to spend another 10 to find it
Good luck, project is looking nice.