Personally, I would first of all fit the rocker back into the pushrod, tighten everything down, check the tappet clearances and see how things go. The torque for the rocker-shaft nuts is:--18 ft lb (25 Nm). The problem with tightening these 2 nuts up with a torque-wrench is the length of the stud above the nut----a 13mm socket THAT long will be difficult to find. You might be able to make up a simple tool utilising a box spanner and, with one end closed up to accept the drive of the torque wrench. I must admit, I have built enough of these engines up that I just do these 2 nuts up--tight! Never (yet!) had one come undone. When you refit the rocker-shaft, make sure that the part that goes over the oil-supply (to the rockers) tube is sitting correctly---I have seen it where the top casting for the tube has not been fitted over the tube, with a consequant loss of lubrication to the rockers.