No worries dude, we all started there at one point. Heck a good portion of overhauling my tractor has been researching how to do this stuff.
Yes to get to the decompression shaft you need to remove the valve cover. On yours you'll need to remove the decomp cable, breather tube, and then the two or three big nuts to remove the valve cover. It's a bit of work but not hard just time consuming. Your shaft appears to be sealed by a single oring where the shaft comes out of the cover (the grove might be in the cover). That oring is probably broken or something has rusted, make sure it's not just the cable before you goto all this trouble. The cams are actually screws going through the shaft retained by nuts and they'll have to be removed to slide the shaft out. You may need to remove the 3 access plates in the cover (two bolts each) to get to the nuts and since they need to come off anyway pop em off and clean the mating surfaces. Once the shaft is out clean it and the end journal (scotch brite works wonders) and replace the oring. You can just match up at the hardware store.
You don't have to be too careful about solvents getting in the crankcase, it's dirt and dust you need to watch, but be careful around rubber seals. Surgical cleanliness isn't necessary but do what you can to clean any dust and dirt you generate out of the inside of the valve cover. Brake clean can be useful but again keep it away from rubber.
If it moves freely now reinstall the shaft (don't forget the oring) and the screws (height doesn't matter just yet), clean out any dirt/dust and then reinstall the valve cover on the engine and reattach the cable. Now it's time to readjust the decompression bolts so they're at the right height. You need to rotate the engine so the exhaust valve on the cylinder you're adjusting is fully closed. Rotate until the rocker is in the top most position and hasn't moved for dozen degrees of crank rotation or so. It's probably easiest to turn the engine using something hooked to the fan belt just make sure the fuel cut off is pulled, don't want thing thing starting on you! Once the rocker is in the top most position pull the release cable and turn the screw so it's just touching the rocker and then go in another 1-1/2 turns and tighten the nut. Rinse and repeat for each cylinder. Once everything looks good apply a light film of instant gasket or RTV to each access plate, bolt them back down and reattach the breather tube.
Thanks again for your great instructions. I'm reading them again now that I've done the other work that was a higher priority for me.
With the benefit of the parts diagram in front of me, this seems much less daunting. There are 3 areas that I'm not quite yet confident about:
1. I assume I need a new gasket for the cylinder head cover and I probably need to torque the two cap nuts to a specific torque. Do I need to oil or otherwise treat the gasket before installing it? I think my WSM omits the engine for some reason so I don't know if it specs the torque.
2. I'm not quite following the part about hooking something to fan belt to rotate the engine. I suppose you are saying to grab it somehow but I'm not sure what you mean by hooking it, and not sure how much force it is going to take.
3. Your instructions sound pretty clear about how to adjust the screws to reach the rocker plus 1.5 turns, but I want to make sure I understand what this is accomplishing. Basically, is what I'm doing setting how much I push the rocker when the cable is pulled? Cable released the valve closes, cable pulled the valve will be open whatever amount corresponds to pressing down on the rocker by the length of 1.5 x the screw pitch minus any play or slop? So the decompress pull just barely cracks open the exhaust valves?
In operation, I need to pull the release hard enough to overcome the valve springs, and when I push back in the release the valve springs will re-close the exhaust valves? The spring on the decompress shaft is just to help it fully return to its rest position and seat the decompress pull handle?