Graham,
I always remove the sprocket cover, (clutch actuator), and the magneto cover. This is the cover to the left of the clutch actuator. The benefit is two-fold.
1) You will be able to see what's going on!
2) It's easier to get a wrench on the nut at the end of the crankshaft, (center of magneto)...and you're now turning the sprocket at a 4.1:1 ratio, (relative to wrench turns), which makes it a lot easier to control the feed of the chain; the sprocket will turn slowly.
I leave these two covers off until final assembly just in-case the chain has to be removed and re-installed.
When replacing the clutch actuator, check for fit. On some of these engines the chain rubs the forward inside edge of the actuator housing. Forward, meaning the surface nearest the front wheel. If it is difficult to re-install the housing screws, file the forward inside edge at an angle to make a chamfer for chain clearance.
Jim