I cant figure out from the drawing how the screw adjuster works but i suppose it somehow turns the actuator, is the clutch cable disconnected from it, try moving the actator arm while attempting to undo this screw. As for the clutch centre nut you will need a tool to hold the thing still while you undo the nut, something like this
or in the past in the days before laser cutting i have made my own out of a bit of flat bar
Its used like this
I assume the engine is stripped and the cylinder is off, with the home made locking device you will need to lock the crankshaft, pass a bar (just smaller than the piston pin and quite long) through the small end eye, place two blocks of wood across the crankcase mouth and allow the crank to turn until the pin rests on the wood blocks. Take great care that the outer drum segment are not unduly strained, theyre easily bent or broken and with such an old bike will be difficult to replace
...Paul